Hello and welcome to Apple Support Communities ChelseaPennant,
I understand that you're unable to make an in-app purchase. I know how important it is to be able to make these purchases, and I'd be happy to help.
Let's have you go in to your settings and try to update your payment information. The article below will guide you through those steps, if needed.
Change, add, or remove Apple ID payment methods
If you go through this article and update your payment information and the issue remains, you'll want to reach out to Apple Support.
Get Support
Thank you for using Apple Support Communities.