I have cash on apple ID won’t let let me purchase for anything

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• if you want to use your Apple ID balance, you need to set your Apple ID balance as your default payment.

See also -> What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

⤵️

• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.