Hi Jabez Dupreez,
I go through the post carefully, based on the error message, the library where save the file could enable Require Checkout files, I would suggest you locate the files from SharePoint libraries, Check out the file before editing and check the result, after finish editing, you may check in the file so that other users can edit the file. For more information and details on Check in/out files in SharePoint, please check the link: Check out, check in, or discard changes to files in a SharePoint library - Office Support (microsoft.com)
Regards,
Clark