How do I fix my card not being able to be updated or added as a payment method? All known tips not working, some side questions unanswered.

I am unsure if expired subscriptions mean that they are unpaid subscriptions, but I recently removed my payment method because I normally do after I finish completing and receiving a purchase. Now all of a sudden that I’ve tried to put my payment method back on it is not being accepted?? No matter what I do I can’t update it or add it.