I've been having this issue since I bought my S7 Edge over a week ago. I am unable to add a payment method or do anything involving payments on both my phone and play.google.com.
When trying to add a payment method, or even redeem a gift card, via play.google.com I get this:
And when I try adding a payment method via my phone I get "We couldn't complete your request. Please retry or try again later." regardless of when or how many times I try.