"…still don't know why that one file won't open.
It likely is time for a new computer but this one works so well."
That one file has been opened by a more current version of Numbers. When openied, the file was translated/converted to a newer version of the file format. Numbers 3.6 is not capable of opening files using that newer file format.
BUT, as SGIII noted above, Numbers 3.6 is capable of opening files saved in the format used by MS Excel.
Open the file on your iPhone or in iCloud, then export the document to an Excel file. Transfer that file to your Mac, and open it in Numbers 3.6. You will lose any features not supported by both Numbers and Excel, but should get a working file (that you should avoid sending to iPhone or iCloud).
Regards,
Barry