User is trying to create time entries for a vendor, but not all vendors are listed in the Employee drop-down on the Time Entries screen.
This happens because, by default, BQE CORE only shows vendors of type Contract Employee and Outside Consultant in the Employee drop-downs on any screen. You can set it to include vendors of type Vendor also by selecting the option Include vendors in employee drop-downs in User Settings > Display Options. Check out this FAQ to know more: Why do vendors show up in employee drop-downs.