it still doesn't work

It works perfectly, if you do it right.

Detail exactly what you are trying to achieve and I can show you how.

Quote from BLOBBYFISH >>

it still doesn't work

 
It has actually always worked. What precisely are you trying to accomplish?

im trying to give my homebrew race ability score increases 

I get that. What abilities, and how big are the increases?

I'm trying to make three options between the following:
+2 Con, +1 Str /// +1 to three select attributes /// +2 to one attribute, +1 to another

I've set each as options under one racial feature "Ability Scores".

I go to character creation, options are available for selection, but only +2 Con, +1 Str affect my attributes.

For the "+2 Con, +1 Str", I've set as flat bonuses with that selectable option.

"+1 to three select attributes" is made with three "choose an attribute to increase by 1" modifiers under one option.

"+2 to one attribute, +1 to another" is made with one "choose an attribute to increase by 2" and a "choose an attribute to increase by 1" under one option.

What am I doing wrong?

You also need to put either 1 or 2 (whichever matches the Subtype) into the “Fixed Value” field for those Modifiers or they won’t work.

This worked, thaks!

Hey I was wondering how to add a stacking Ability Score modifier AND an Ability maximum Score Modifier that increases/changes per player level. I'm trying to make a Giant-Kin Race that gets stronger as it levels up and without just making a whole bunch of feats.

Quote from cheezygringuy >>

Hey I was wondering how to add a stacking Ability Score modifier AND an Ability maximum Score Modifier that increases/changes per player level. I'm trying to make a Giant-Kin Race that gets stronger as it levels up and without just making a whole bunch of feats.

 
You unfortunately cannot.

Damn

Is there a way to make so you choose one ability score to increase by two then the other increases by one. For example: 

"Choose either Dexterity or Charisma. That ability score increases by two, while the other score increases by one."

Yes. You would create two Options attached to that trait, one that has the Modifier to boosts Dex by 2 and the Modifier to boost Cha by 1, and the other Option that has the opposite Modifiers. You can learn all about Options in the Homebrew Subclasses & Species FAQ, question #1: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq#HomebrewSubclasses&SpeciesFAQ). 

It is not entirely clear how to make a fixed attribute value for a race.
One of my players became a vampire and I want to be able to change his race through hombrew.
However, if I set it through a modifier like:
Modifier -> Set - Charisma Score Fixed Value =18, it did not work at all.
Any ideas? Or is it impossible to do this?

Is there a way to make an ability score +3?

Quote from Audrielle_Winslet >>

Is there a way to make an ability score +3?

 Set the fixed value in your bonus to 3.

Quote from Kailaidescope >>

Quote from Audrielle_Winslet >>

Is there a way to make an ability score +3?

 Set the fixed value in your bonus to 3.

 I did that and it worked thank you!

Im trying to figure out something similar. I have a transformation option on a species that changes all of the characters original ability scores to specific numbers, but when using the Modifier Type of "Set" and then the number I want, nothing changes at all

Quote from Zentrios >>

Im trying to figure out something similar. I have a transformation option on a species that changes all of the characters original ability scores to specific numbers, but when using the Modifier Type of "Set" and then the number I want, nothing changes at all

 
Ability Scores "Set" works a little differently than other "Set" things. It cannot make an ability score lower. If they had, for example, a Strength of 16 and your tried Set to make it 13 - it will not work and the score will remain at 16. But if their score was 10 and you were setting to 13, then their score would become 13.

Using Bonus and a negative value in Fixed Value will work to lower a score but, of course, this means if they ever increase their score through an ASI you would need to update the homebrew feature and change the fixed value appropriately. Theoretically you could use this to decrease the scores by say 30, which guarantees the score goes down to minimum of 3 no matter what it is, and then have Set to the value you want. I have not tried this, however, so may require some experimenting. Let me know if it works!

EDIT

So was curious enough to test myself. I made a feat that first reduced every score by 30 (Bonus > Relevant Ability Score > Fixed Value -30) and then used Set for every ability score to be 10 (Set > Relevant Ability Score > Fixed Value: 10). And it worked: all scores became 10 no matter what they were before, for any character I added this feat to.

Although I used a feat for convenience this can be replicated using homebrew creator for species, subclasses or magic items.