I am looking to make a homebrew race that with each level up, it's constitution score increases by one, so besides whatever stats the character rolls, say for instance I roll a 10 for their con, each level increases it by one. So level 1 is 11, level 2 is 12, level 3 is 13, and so on.

You would probably just have to make a feature for each level from 1 to 20 which increases the Constitution Score by 1. Just make sure to tick the 'hide in details page' for the last 19.

Basically, once you've created the race, go to 'Species Traits' and add a trait for each level. Just make sure to tick the 'Hide in details page' box (for every level except 1st) and write 1, 2, 3, 4 and so on in 'Required Character Level'. Then once you've made the trait, go to Modifiers ---> 'Modifier Type' = Bonus ---> 'Modifier Subtype' = Constitution Score ---> 'Fixed Value' = 1. This means that once that feature is achieved, the Con score will increase by 1. Now just repeat that for every level, one through twenty, and you're done. That's the only way I can think of of doing it.

So I tried to do this, but for some reason it is adding all the bonuses to the con score even at level 1, and I set it to only add at specific levels. Below is an image of what I have set up. 

I believe you have to leave the 'Primary Stat' empty. Not set to CON. There's no reason to set it to CON since you're already adding a bonus to the Con score with the modifier. Just set 'Primary Stat' to nothing and see if it works.

I tried that, but it still just adds all the bonus all at once instead of adding with each level