Won’t let me buy anything on my iTunes says a problem with a previous sale, but won’t let me change my credit cards

What do you mean 'won't let me change my credit cards', what happens ? When you owe iTunes money you should be able to change to another card (as long as it's a valid card) : If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - Apple Support

Is the debit card registered to exactly the same name and address (including format, spacing, abbreviations e.g. 'St' instead of Street etc.) as on your iTunes account ?

bought a prepaid iTunes card and it’s on there but won’t let me use it !!

What happens when you try to buy something, and do you still have the invalid debit card still on the account ? If you do still have the debit card on it see if you can remove it (when you have a card on an account it has to be valid and accepted by iTunes, even if your account's balance covers what you are trying to buy) or replace it with a different card : Change or remove your Apple ID payment information in the iTunes Store - Apple Support