I'm working on a custom race that would grant expertise with one item or skill of the player's choice at level 3, be it a weapon, tool, or armor type, but when I go to test it in a character it doesn't give me any options to choose from. 

You got two issues the first is that you don’t want to give “expertise,” you want to give “twice proficiency.” Expertise is for things your character is already proficient in, it just bumps up how proficient. But twice proficiency lets be “anything,*“ and if they pick some they had already chosen as a standard proficiency in cui hey can go back and choose something else.

The second problem you’re having is, there is no such thing as twice proficiency with weapons* in 5e, that’s just not possible. It’s not in the official rules so DND abeyond does not have code for it.

oh, i see.  well, in the case of the weapons, is there an alternative you might suggest? the race in question  is meant to be one that gets extremely good at whatever they choose to do, which is why I'm giving the option to choose and expertise in the first place, as a way to expand further on whatever they choose to specialize in with their class and fighting style

No, there is absolutely no mechanical way to grant a character twice proficiency with any weapon on DDB’s system.

The homebrewer… it isn’t actually a homebrewer at all. It’s the exact same system the Devs use to input the game’s official publications from WotC. Since WotC has decided they they will not be including anything that would do that ever this edition,* DDB will not program that function into their system.

The only conceivable thing you could do would be to include the information in as plain text. The players would have to not select anything from the dropdown, basically leaving it set to “—“ instead ot sel and then manually adjust their Attack modifiers for weapons of the chosen type directly on their character sheets by “Customizing the attack and adding 2-6 to the attack bonus, depending on their character’ scurrent Proficiency bonus. 

*(it would mess with the bounded accuracy too much. That’s basically the bubblegum that holds this entire edition’s combat system together.

Makes sense, and im only just now realizing that expertise in a weapon would be overpowered, and that expertise in an armor type is redundant, as proficiency in an armor type only grants you the ability to use that armor type outright

Quote from Alexlockstone12 >>

Makes sense, and im only just now realizing that expertise in a weapon would be overpowered, and that expertise in an armor type is redundant, as proficiency in an armor type only grants you the ability to use that armor type outright

 If you have access to Xanathar's, go into Homebrew and create a new Feat by copying the Prodigy Feat. It will give you ideas on how to set it up as an Option on the race when they reach Level 3 for Tools and Skills.

Yup

ALeo (and I know this will seem nitpicking but it is an important distinction) Expertise ≠ Twice Proficiency. They are related, but not the same. If something grants Twice Proficiency in something, it is just like granting regular proficiency in that thing, except instead of adding PB to those checks, it adds 2×PB.

Proficiency Bonus

(https://www.dndbeyond.com/sources/basic-rules/using-ability-scores#ProficiencyBonus)
Characters have a proficiency bonus determined by level, as detailed in "Step-By-Step Characters." Monsters also have this bonus, which is incorporated in their stat blocks. The bonus is used in the rules on ability checks, saving throws, and attack rolls.

Your proficiency bonus can’t be added to a single die roll or other number more than once. For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once when you make the save.

Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. For example, the rogue’s Expertise feature doubles the proficiency bonus for certain ability checks. If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you still add it only once and multiply or divide it only once.

By the same token, if a feature or effect allows you to multiply your proficiency bonus when making an ability check that wouldn’t normally benefit from your proficiency bonus, you still don’t add the bonus to the check. For that check your proficiency bonus is 0, given the fact that multiplying 0 by any number is still 0. For instance, if you lack proficiency in the History skill, you gain no benefit from a feature that lets you double your proficiency bonus when you make Intelligence (History) checks.

In general, you don’t multiply your proficiency bonus for attack rolls or saving throws. If a feature or effect allows you to do so, these same rules apply.

Proficiencies

(https://www.dndbeyond.com/sources/basic-rules/personality-and-background#Proficiencies)
Each background gives a character proficiency in two skills. Skills are described in the Using Ability Scores section.

In addition, most backgrounds give a character proficiency with one or more tools. Tools and tool proficiencies are detailed in the Equipment section.

If a character would gain the same proficiency from two different sources, he or she can choose a different proficiency of the same kind (skill or tool) instead.

Whereas “Expertise” on the other hand allows a PC to double their proficiency bonus to a Skill or Tool they are already proficient with.

Skill Empowerment:
Your magic deepens a creature’s understanding of its own talent. You touch one willing creature and give it expertise in one skill of your choice; until the spell ends, the creature doubles its proficiency bonus for ability checks it makes that use the chosen skill.

You must choose a skill in which the target is proficient and that isn’t already benefiting from an effect, such as Expertise, that doubles its proficiency bonus.

Expertise (Bard)

At 3rd level, choose two of your skill proficiencies. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

At 10th level, you can choose another two skill proficiencies to gain this benefit.

Expertise (rogue)

At 1st level, choose two of your skill proficiencies, or one of your skill proficiencies and your proficiency with thieves’ tools. Your proficiency bonus is doubled for any ability check you make that uses either of the chosen proficiencies.

At 6th level, you can choose two more of your proficiencies (in skills or with thieves’ tools) to gain this benefit.

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 The same also goes for these; but they are not free access content so I’ll simply link them instead of excerpting:

Artificer Class- Tool Expertise; Feats- Prodigy,  Skill Expert; Optional Class Feature- Bardic Versatility.

One analogy that often helps is thinking of Twice Proficiency as getting a brand new, bleeding edge computer, Expertise is like taking whatever ‘puter you already have and upgrading the 🐂💩 out of it until it’s the next best thing to Twice Proficiency.

So… which are you trying to include: Twice Proficiency, or Expertise? Is this an upgrade to something they already have or is it newnew/replace&add? As I’m you said, Expertise (or Twice Proficiency) in Armor would be pointless, and I with weapons it would break the bounded accuracy (so hard).

yes, which is why i made it expertise in the first place, because the whole point is to specialize in pre-existing talents and proficiencies, and so they couldn't just choose some random,, unrelated skill or tool.

If you use the following, it should let them pick any Skill, or Tool*1 they are proficient with. That’s about as comprehensive as you’re gonna get within the system. 

• Modifier: Expertise->Subtype: Choose a Skill or Tool

The only reason the weapons and armors are even in that list is because the system wasn’t really designed for us (site users), but for the Devs themselves (we just get limited access). The Devs get lists of what does or doesn’t work to reference if they don’t just know the stuff, so it was faster and easier (and cheaper) to just use the same list of Modifier Subtypes for all five*2 of the proficiency related Modifiers. It was also faster/easier/cheaper to not bother programming the subtype dropdown to hide options that don’t work for the modifier selected.

1. *(“Tools” include all Artisan Tools, Non-Artisan Tools,*3 and I believe Vehicle too.)
2. *(“Half Proficiency,” “Half Proficiency Rounded Up,” “Proficiency,” “Twice Proficiency,” and “Expertise”)
3. *(Disguise Kit, Forgery Kit, Herbalism Kit, Musical Instruments, Gaming Sets, Navigator's Tools, Poisoner's Kit, and Thieves' Tools)

oh nice, it worked!