Hello, Imitation_co!
From your post, it sounds like you are trying to add funds to Apple Cash without success. We’re glad that you reached out to the Apple Support Communities. We want to try and help you find the answers you need.
Have you already set up Apple Cash? If not, follow these steps: Send and receive money with Apple Cash. As it shows there, you can only add funds to Apple Cash using a debit or prepaid card. "You can't use credit cards with Apple Cash, and you might not be able to use some prepaid cards. You can't add money from a gift card, even if it's already in Wallet."
If you need help with adding a credit, debit, or prepaid card to the Wallet app, follow the steps on this guide: Set up Apple Pay. If you don’t already have one of these cards in Wallet, you'll need to follow those steps to add one.
We hope this information helps you.
Have a great day!