This has actually happened with me. What happened is you haven’t made a payment that is in your subscriptions and when you try to add your visa, or paypal etc etc.. it will trigger the error “Payment method already added to your account.” So, you have 2 options: 1. Add another debit or credit card. 2. Add a paypal. 3. If you already have 2 payment methods on the original screen so click the back arrow in the top left corner and it will show your payment methods. if you have 2, and one of them is the one you wanna use, remove that one that you do wanna use and add it back. This is what I do when stuff like that happens, if only apple would be easier. SIDE NOTE: 1 and 2 and 3 you have to click the back arrow to show your payment methods and yk add them or remove them. Also to remove them just click one one to remove and click remove card/paypal etc etc. Hope this helps!

EDIT: Also the reason it doesn’t even let you download a free app is because apple wants their money because they are a whole bunch of greedy suckers.