It doesn't really make sense to change the location to buy a game.
If your country is supported by Stadia, you should have no problems to buy and play games on Stadia.
If you are living in an unsupported country and try ways like a VPN or something to be able to play Stadia, then it might be the case that you can't buy and play every game on Stadia or it can also happen that the complete service will not work you anymore one day.
Another reason might be that you have a child-account (if you're under a certain age) and your account has restrictions regarding certain games and that's why you can't buy it (don't know if that's the case for you, was just an idea).
