Have you included the division number, even if you do not use divisions you must import 00 which is the default division.

Yes the Division is include.  Out of 2,400+ records on the import file, this is the only one with this issue and is the only record for this vendor.

Is the vendor inactive?

I would look at the source file, look at the record BEFORE this record. I have seen where there is an error on the record before that affects the next record.

Vendor is active

I also check the vendor for the previous and following record.  Both are valid.

Check for a blank character at the end of the Vendor# in AP_Vendor (which can be hard to do sometimes...).  An import requires an exact text match within the source data... and within the Sage UI this will be auto-filled so it's easy to miss.

One way to test is to create a source file with just that one record. Import. If it goes in o.k. then something is wrong with the format of the record before it. If it crashes on that one record something is wrong with the format of that record.