Play Store - "An unexpected error has occurred. Please try again later." 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.
So it turns out the Google Organisation my Google Account was managed by had Google Payments (formerly Google Wallet) disabled for all users; after enabling it, everything works perfectly.