What Cash App? This one?: https://cash.app/
If so, that has nothing to do with Apple, but if you tried to buy something or started a subscription or free trial and did not cancel it, it may have renewed and tried to charge you through the payment method added to your Apple Account. If the payment method was this cash app card, then it would be unable to charge you if its locked.
You need to unlock it, or add a different payment method to your Apple account to complete the charge, whatever it may be.
You can verify what pending charges you have on your Apple Account by following the steps in the support document linked below.
click here ➜ View your purchase history for the App Store and other Apple media services - Apple Support