The following are the most likely reasons why users are not able to register for Eight Premium:
Different Apple ID or Google Play account
If your device is logged in to a different Apple ID or Google Play account from when you downloaded Eight, you will not be able to register for Eight Premium.
Either log your device into the account used to download Eight, or delete Eight (be sure to remember your login information so as not to lose your data) and download it again with your new Apple ID or Google Play account.
Invalid Payment Method
Monthly or annual subscriptions cannot be paid for with gift cards. You need to pay with a credit or debit card.
Outdated App Version
Please make sure your version of Eight is up to date.