I'm currently trying to add a modifer to my subclass options but it's not working?

Basically I'm trying to program in, "You gain proficiency in Perception. If you already have proficiency in Perception, you gain expertise."

I've tried stuff like setting my modifier to proficiency, twice proficiency, and expertise and sub-modifiers to choose rogue expertise, and perception, but nothing is working how I want it to.

Is there a way to do this? Do I have to create mutiple modifiers and do something with that?

If this is for a homebrew subclass maybe the homebrew forum will get you the answers you are looking for. Also, I know one homebrew change I tried took a while to go into effect. I made the change and saved. And when I went to the subclass to see if it worked it didn’t (I thought). The next day when I went back to it, the change had taken effect at some point. 

The "if you don't have it, gain proficiency, if you do get expertise instead" is not possible. IF statement operation is not supported.

You will need to set up Options, one for Proficiency and one for Expertise. The user will need to select which is appropriate.

Modifier: Proficiency -- for the Proficiency option, Modifier: Expertise -- for the Expertise option.
Modifier: Subtype: Perception.