"CUSTOMER X was used to report time. You cannot delete this project."
When deleting a project, you may encounter this error if the project was used to report time.
Make project inactive
Instead of deleting the project, a better solution might be to inactivate it - especially since you may need to keep the time data for historical data or audits.
Delete all time, then delete project
If you are working with a Unanet system that has yet to go live (during implementation, for example), you can consider the option of deleting all time and then proceeding to delete the project.
Note that time might be LOCKED or EXTRACTED, and therefore cannot be deleted via the user interface or via import. This would mean the project also cannot be deleted. In this case, you could consider making the project inactive when it is no longer needed. Otherwise, you could request a script from Unanet Support to delete the time.