The Reporting Manager should also be a TT User in the org.
Please create a TimeTracker user by following the instructions provided here.
Once the user is created, go to the PK4 Licenses tab and assign them a TimeTracker Manager license by following the instructions provided here.
Once you finish creating the TT user and have added the TimeTracker Manager license, you can now add the user as a Reporting Manager to other users by following the instructions here.