If you have purchased a Premium membership and appear as a free member, first click on the "premium member" button on the main page and press the "restore my purchases" link at the bottom of the purchase page that opens.
If you still get the "used by another account" warning and still won't unlock, you may be experiencing this issue for the following reason:
All Premium Memberships on Nowanda are connected to the email account from which you made the purchase.
Suppose you log in with a different email or an account like Apple, Google ID, or Facebook. In that case, the system will recognize you as a free member if your email address in these accounts differs from the account you purchased.
First, click on the "Other" section of the application and check which account you are connected to Nowanda from the My Profile tab, and make sure that you have purchased a Premium Membership with this account.
If you find that it's a different account, click the "Sign Out" button at the bottom of the same page and close the application.
Reopen the application and log in with the account that your Premium account is linked to. Your locks will open.