There is no error message and nothing to show in a screenshot. When I send an email with an attachment, the recipient says, "You forgot the attachment." But I did not forget the attachment. I can see it attached in the Sent Folder.
It is an intermittent problem and makes Apple Mail unusable.
See this link to understand that it's an ongoing problem: Mac mail app won't send attachments with … - Apple Community
Here is what I have tried - without success:
1) I've tried both insert options available through the app - file/attach files and the paperclip icon
2) Go to Top bar > Edit > Attachments > Unselect this "Always send Window-friendly attachment"
3) Made sure 'Include Attachments' is checked in the tool bar
4) Open mail app select Mail>Settings>Privacy disable all:
Protect Mail Activity
Hide IP address
Block all remote content
None of these things work.