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

1. Open Lutris & Click the + icon (top-left)
2. Select Add Locally installed game
3. Go to Runner options tab
4. Finally you'll see wine version, Select Proton - Experimental

12USD for a game with frequent content updates, by an indie dev. Pirating this is just spiteful.

Agree; however, I do support it being available for people who wouldn't be able to afford that, even if 12USD doesn't seem like much often that's all they have and would probably be better off buying food with it. This way they can play it on their shitty laptop that was given to them by some charity and not starve to death. In those circumstances, it might just be the only thing that keeps them alive.

I also support piracy for games that don't have functional demos, not sure if stardew valley has one.

And for people like kids and students without much disposable income. I pirated a couple in that time, and bought many whenever I had the income.

Also for singleplayer games with anti cheat, lmao get stuffed.

Everything on steam has 2 hour demo. And if 12usd is too much, the android version is a little over $3.