It appears that during the upgrade or initial installation, the TimeTracker app was configured to be visible only to specific Profiles within the Org. 

You can grant access to the Tab by following these steps:

  • From Setup,
    • Enter Permission Sets in the Quick Find box, then select Permission Sets, OR
    • Enter Profiles in the Quick Find box, then select Profiles
  • Select a permission set or profile.
  • Do one of the following:
    • Permission sets or enhanced profile user interface—In the Find Settings... box, enter the name of the tab you want and select it from the list, then click Edit.
    • Original profile user interface—Click Edit, then scroll to the Tab Settings section.
  • Specify the tab settings, Permission Set with Salesforce License
    • Create a Permission Set  (Select the license as Salesforce and  Save the Permission Set) or Modify the existing one.
    • Click on 'Object Settings
    • Click on '<TT Objects>' Object
    • You will find that Tab Settings is available
  • (Original profile user interface only) To reset users' tab customizations to the tab visibility settings you specify, select Overwrite users' personal tab customizations.
  • Click Save.

Now the users will have access to the TimeTracker objects.