I redeemed an apple gift card and now I want to add it as my payment method but it won’t let me

Did you make sure, that the Apple ID payment is listed on top of your payment methods, as shown in this article?

Change, add, or remove Apple ID payment methods - Apple Support

If it is not at the top of the list, can you move it manually to that position?

Yes, and it is, but when I try to make a purchase it continues to ask me for a payment method. I don’t have a credit card linked to my other accounts and have no issue.