I have a homebrew race which I'm trying to give advantage on saving throws against charm and outright immunity against sleep due to Fey Ancestry but the option for that doesn't seem to exist. Could someone explain how I could do this?

Its a little janky in the tool right now. The Details field for modifiers is also the restriction. So you use

Modifiers->Type: Advantage, Subtype: Saving Throws, Details: against being charmed, and magic can't put you to sleep.

I thought Charm Resistance gave Advantage to those Saving Throws. Wouldn’t it be “Resistance: Charm” instead of “Advantage: Saving Throws” in the builder? If I’m wrong please lemme know.

Resistance is used for damage calculations and halves the damage received from the indicated sources. Charmed does not deal damage.

Fey Ancestry: You have advantage on saving throws against being charmed, and magic can’t put you to sleep.

You can spot check how D&D Beyond does this by creating your own copy of Elf and looking at its Fey Ancestry trait.

Cool. I thought I had remembered them doing it as a “resistance,” but I stand corrected. Thanks. 

Quote from IamSposta >>

Cool. I thought I had remembered them doing it as a “resistance,” but I stand corrected. Thanks. 

 
You may be thinking of the Magic Resistance trait which doesn't act as a resistance but just gives advantage to saving throws against spells and magical effects (see Yuan-Ti Pureblood race). 

That could have been it. As my mother always says: “The day you stop learning is the day they throw dirt on top of you.” I try to not stop learning.

Quote from IamSposta >>

That could have been it. As my mother always says: “The day you stop learning is the day they throw dirt on top of you.” I try to not stop learning.

 
She seems wise.

Thanks. Any idea how to do disadvantage on grapple checks? As the grapple option doesn't seem to exist.

You can do

Modifiers -> Type:Disadvantage, Subtype: Ability Checks, Details: vs grappled

But it won't show up on the sheet anywhere. There is not a place for ability check modifiers.

Damn. I was trying to have everything on the sheet so I don't have to waste time explaining things to my DM. I might just leave that out.

Quote from Cyb3rM1nd >>

Quote from IamSposta >>

That could have been it. As my mother always says: “The day you stop learning is the day they throw dirt on top of you.” I try to not stop learning.

 
She seems wise.

 She is.

Since grappling only ever RAW involves Strength (Athletics) and Dexterity (Acrobatics) checks, you could add Disadvantage to just those two with a stipulation.
Modifiers -> Disadvantage | Modifier Subtype: Acrobatics | Details: Grapple Checks (or however you want to word it)
Modifiers -> Disadvantage | Modifier Subtype: Athletics | Details: Grapple Checks

Whenever you click on those abilities in the character sheet, it will have that stipulation show up.