User is trying to create a time entry on a project, but the activity item shows up as non-billable by default, while it is marked as Billable in the Activity Items screen.
This can happen if the activity item is assigned to the project in the Projects > Assignment screen and its Billable flag is unchecked there (overwritten). To make it billable, you need to check its Billable flag in the project assignment. Check CORE Help Center for details.