. . . on my cashapp card
Are you talking about this app ?
You can't use Cash app as your Payment method for the App Store.
See -> Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.
If you don't have a credit card or don't want to use a credit card, you can also use your Apple Account balance as payment method.