If you are seeing that error message, it is because you may currently be registered under two different accounts: one for IMDb.com and one for IMDbPro.com. This probably happened because when you subscribed to IMDbPro.com you used a different email address than the one you used when you registered with IMDb.com, which resulted in the creation of two different accounts. Maybe you had an old IMDb.com registration in the past and there's still and old cookie in your computer (for more details about login problems, see also our Cookie FAQ.)
The above error message is being displayed because the computer you were using when you tried to access IMDbPro.com is already logged in at IMDb.com under the incorrect account (IMDb.com registration doesn't allow access to IMDbPro.com). In other words, you are trying to access IMDbPro.com using the login information of a free IMDb.com account.
It's like you were trying to board a plane using a train ticket because you took it out of the wrong pocket. Just show the correct ticket and the man at the gate will let you in. You need to logout and login again using the correct email address (the one you chose when you subscribed to IMDbPro.com).
The easiest way to do this is to perform the following two steps:
If you do not have a subscription to IMDbPro.com, you can sign-up for a 14-day free trial.