When you attempt to log off a user session via terminal services manager or via task manager, you receive the below error:

Error:User ‘username’ (sessionId=) could not be logged off. Access is Denied.

The above error occurs when you do not have correct permissions. Ensure you have the correct permissions or are part of the correct security group.

Logoff and back on after correct permissions have been granted. You should be able to logoff sessions without the above error.