storage server credentials

My iCloud storage is almost full. I am trying to upgrade it to the 2TB, however, I can't access the purchase upgrade page.

On the iCloud management box on storage info it says 'Details not available' by the manage storage information bar (which is grey).

When I click upgrade it says 'An error occurred validating your storage server credentials. Please try again later' I tried the next day and have the same issues. How can I fix this please?