Which version of Payroll? Any recent updates?

Did you check out Solution ID:  223924950047292?

We use PayMate for payroll. Paymate creates an export file that I save on my server and upload using VI into my GL in a JE entry.

So you are importing into the GL (without using the Sage module for payroll).

Why is the header failing?  There should be another error in the job log for that.

Wild guesses: source file format could have changed... or if you newly turned on batches in GLJE you'd need to add BatchNo to the VI job.

This is the error:

and

The Line is always going to fail if the header fails, so don't worry about the line until you resolve the header issue.

Is there a Source Journal of "P"? 

I've never seen a 1 character source journal, although that doesn't mean it's not possible - just not standard practice.

Edit: Guessing that really should be "PR" 

Indeed.  Source Journal Maintenance must have a record matching the code used in a GLJE record.

Yes, it should be PR and it has always been PR.  Where do I fix this?

The VI job settings should show you how this value flows into the import.  If you don't know what that means, it might be a good idea to ask whoever manages your Sage 100 system.