Hi,
Prima facie Check your User card set up //Departments->Administration->Application Setup->Users->User Setup->User ID
and check the user card->General Tab
As Natalie and Alex are pointing your license can have 5 full users and 1 limited users
Then Check for the Sessions open (How many users are connected to NAV)
Dont forget to save your Licence on database //Development Environment-> File ->Database->Alter->Integration (tab)-.>Save License in Database
Lastly restart the Service from NAV administration tool and check again..
Thanks,
Anjan