User cannot enter time on a project when the total amount charged on the project has reached the contract amount.
This happens when the project rule Prevent time and expense entry after reaching contract amount is set in Project Details > General > Assignment screen. This rule prevents you from making billable time and expense entries after the total amount charged on the project reaches the contract amount. However, you can still make non-billable and extra entries on the project. Check CORE Help Center for details.