Hi M Nicoletti,
Thank you for posting your question in this community. From your description, it seems that some of your user cannot add the file from SharePoint document library into their email which they didn’t have certain
options. May I know how did your members add SharePoint files as the attachments in Outlook email?
Generally, we can add the SharePoint file as an attachment in Outlook by clicking on Attach > Browser from Cloud location > Groups > Then choose the file from a document library.

For the second question about the error message when the user send email attachment, may I know where did she get this error message, in SharePoint document library or Outlook application? If she attached the
file in email from Outlook desktop application, I suggest you can let here try to attach the file in SharePoint document library with
Outlook for the web application
and see whether the same error happens.
Best regards,
Dihao