There have been multiple reports of errors with the Flat.File.PriceInventory.TTH file on Excel for the Mac since 2014, and it doesn’t seem like Amazon want to do anything to fix it. Visual Basic Unexpected Error 465 prevents users from saving the file after making edits.
A quick workaround for this is to use Google Sheets. Import the downloaded Price & Quantity Update Loader File, make your updates through your browser on Google, then save it to your computer as a tab delimited file.
Change the file extension from tsv to txt, and upload to Amazon, making sure you specify that you’re uploading a Price & Quantity Update Loader File from the drop down menu there before you upload it.
After many hours, this has finally fixed the upload issue.