If you download Stardew Valley from Fitgirl you might encounter mentioned issue. Seems like Proton-GE no longer works.
If you facing issue like this just use Proton - Experimental.
Guide
- Open Lutris & Click the
+ icon (top-left)
- Select
Add Locally installed game
- Go to
Runner options tab
- Finally you'll see wine version, Select
Proton - Experimental

I get where you're coming from. I'm actually one of the lucky ones. I've got a multi-currency visa card and I did buy the game (for 5 dollar from steam). In my hall, one of my friends wanted to try this game so he pirated it and ran into this issue. That's why I'm sharing the solution here.
But here's the thing most students in my country (south asia) can't drop 12 USD on something. That's like 1400 bucks in our currency which is enough to cover meals for 3 days as a student. And even if you've got the money there's another problem. Platforms like Steam straight up reject our cards even multi-currency ones. I don't blame them but it makes things impossible. So it's not just about affordability but also the availability. It's not like we don't want support anyone we do but sometimes the circumstances aren't on our side.