Every Sky Stpre account has a spending limit on it so what might have happened is that yours is set at £0 meaning this error will occur.
I would get in touch with Sky Store support regarding this, they have a live chat you can use that is available from 11am-7.30pm each day and can be found here:
https://www.skystore.com/help/contact-us
__________________________________________________________________________________________
If someone has helped you then please click on the LIKES button in their post.
If you need help please provide as much information as you can
