Why can’t I purchase anything in my game?

If in-app purchases are working in some apps, but not others, that would suggest that you have a valid payment method on file.

If your iPhone is subject to Parental Controls, your parents or guardians might have set things up to block in-app purchases, or to allow them only with their approval. I don't think the phone or the App Store would be making a distinction between in-app purchases for games, and in-app purchases for other apps.

But, of course, if your phone was subject to Parental Controls, your parents could be making the decision to allow some types of in-app purchases and not others.