This can happen because of two reasons.
- First, check the project's contract type. Core automatically labels Overhead projects as non-billable. You can do this by going to Projects > General > Details and check the contract type.
- If this is not the issue, then you might have exceeded your contract amount for the project. If that is the case, you can either increase the contract amount for the project or you can select the rule 'Skip automatic time entry evaluation' in the Projects > Assignments tab.
If you are still unable to change the Billable status of the time entries associated with the project, try doing a Batch Update on your time entries to mark them as billable manually. Check Core Help Center for details.