In AP - Tools - Import Invoices, Click on the Import Format and click Preview for another example.  At line one, the record starts with API for the invoice information and the distribution of that invoice starts with APD.  Below is an example of an import file you can see in AP - Help - Help Topics, the index tab under Import Invoices - File Example: 

I think your file may have started with ' ' instead of API.

Thanks for the reply.  I had that as per instructions, but FYI I finally figured out the problem.  The invoice data was imported from a Sage50 export to excel.  I added the first column for the API/APD identifier.  All seemed correct so I could not figure why it was not accepting this.  I opened a new excel file and did a copy and special paste, data only, and it imported without a problem. Some strange formatting must have come with the original excel file created from the Sage50 export..