When you log in you will get this message if you account isn't attached to a client account.
Users have just one account on Sheep regardless of the number of 'flocks' (client accounts) that they are a member of. Most people are only a member of one but a few exist on multiple systems.
Accounts will become orphaned if the user has their account removed from a flock and they are not a member of any other account. User access and permission is controlled from the 'User Permission' sub-page of Settings.
How to fix
Ask your administrator to re-attach your account (they will need the email address of your Sheep account). If you don't know who your account administrator is then please contact firstname.lastname@example.org