SUMMARY:
There are scenarios when you have both "Project and Billing" and "Time & Attendance" licenses enabled in the application.  You may plan to discontinue using "Project and Billing".  So you create activities and allow users to enter time on activities.
 
However after the "Project and Billing" license expires users do not get an option to enter time.  They do not get an option to add activities to their timesheet.
 
SOLUTION:
This can happen due to permissions assigned to the users.  Please follow the steps given below to resolve the issue.
 
In the permissions assigned to the user, If "Against projects" is selected for "Allow time to be entered" drop-down then this issue may happen. 
 
To check the permissions assigned to users: 
 
1. Login with admin credentials 
2. Click on the administration icon on the top 
3. Click on "Permissions" link in the "Users and Departments" section 
4. Double-Click on the name of the permission 
5. Click on the '+' symbol next to Timesheet permission. 
6. Change the "Allow time to be entered" permission from "Against projects" to "without requiring a project to be selected" 
7. Save the permission 
8. Follow the same step on the other permissions assigned to the user. 
For example if a user is assigned two permissions, TimeSheet and "Exempt – Time and Time Off" then you need to check both these permissions. 
 
(please check the print-screen "Without requiring a project to be selected.PNG" attached to this Solution)