This happens if the Time entry option is disabled for the task added to a project or if the user is not added to the task.  Follow the steps below to allow Time entry for the project added:

–  Login to Timesheet with Administrator Privileges.
–  Click on Project Icon.
–  Identify the Project for which the task is added.
–  Click on Edit button.
–  Click Edit under Project structure on the right hand side of the screen.
–  Check the box for Time entry allowed, also select the employee's name (for whom the access needs to be 
    given) under assignments and move them to allowed list.
–  Click Save to apply changes.