contact Microsoft for Store issues, i got a similar issue with an other game and they should be able to help you. Every transaction in the Store is going thru Microsoft and very often it's related to "region" issues. For example you account is based in europe and you console is set to the US. In my case, i work/life in two regions, i was required to add a credit card to both regions and i believe it could be something like this. MS support will tell you...
Cheers
Michael
Playing on: XBOX One - European Server
Side: Aldmeri Dominion
Location: Germany/Switzerland
I have credit on xbox that I cant use on eso premium. Whats going on?
Microsoft do not allow the use of store credit for subscriptions. This applies to Xbox Live Gold, EA Access and also ESO Plus. These are subscription services that are generally recurring after each 30 day period so in order to purchase them you need a credit/debit card.