This is typically a Payroll Code issue - please check the following:
1. Payroll Code Defaults in settings (settings -> Payroll -> Default Payroll Codes)
2. Jobsite Type Payroll Settings (settings -> Job Types -> Default Payroll Codes for Each Type)
3. Job specific Payroll Settings (Jobsites -> Specific Job, Payroll Tab)
Chances are someone has changed the list of payroll codes (or deleted a code in use somehow), and you'll see a blank dropdown somewhere in the above steps.
If this does not fix the issue, please contact LMN support at firstname.lastname@example.org.