Issue is likely caused if the user is part of the project team, but not part of any of the tasks. If you have enabled 'Allow Time Entry against Task Only' in the project info page as shown in the below screenshot, then follow the steps provided to resolve the issue:-
1. Log into Replicon as Project Administrator. 2. Go to the Project Management tab. 3. Go to the concerned Project > Tasks Tab. 4. Click on 'Edit Task'. 5. Click on drop down for 'Change Resource Assignments'. 6. Put a check box for the required user. 7. Click on 'Done' and 'Save'.