SharePoint allows file attachments to lists. SharePoint stores these attachments under URL/SiteName/ListName/Attachments/#/filename.ext where the # is the ID of the list item with the attachments.
When connecting Tableau to a SharePoint site, one of the tables presented to Tableau is "Attachments" which is a reference/table lookup SharePoint uses to find the attachments. Tableau does not handle this Table correctly. It complains about missing fields and is unable to pull information from it. Tableau can pull information correctly from other lookup tables (like the permission tables for example)
When connecting to the actual SharePoint list with the Attachments, the Attachment field is simply a Boolean Y/N field without any information about the attachments.
I suggest Tableau either fix the interaction between the "Attachment" lookup list or some other method to lookup the attachment files on the SharePoint list (there is a direct REST API that tells you).