User is trying to save a time entry, but is prompted that the operation is restricted by project assignment.
This happens when a time entry has an incorrect combination of project, employee and activity, which is not in accordance with the specified project assignment. For example, you might be trying to use an activity in the time entry that is not assigned to the project. You can review the assignments for the project by going to Projects > Assignments. Select the project whose assignments you want to review and make sure that you add the employee (Team Member) or activity to the project that is missing from your combination of employee and activity for the time entry. After you assign the missing item, Core allows this time entry.