If the system does not recognize your email address it can mean that :
1) You have used another email at registration
2) You haven't created an Ultimate Guitar account yet
In the first case, please try all your email addresses that could be used to create an account. The forgot password or username instruction could also help you.
In the second case, please create a UG account (it's free):
1) Open the app
2) Go to "Account & settings"
3) Tap "create account".
After your created your UG account, your subscription will automatically attach to it.
Note: Purchasing a subscription does not create a UG account for your automatically. Your subscription is attached to your Apple ID account and your UG account should be registered in the app.