Mail app has gone bad

I can no longer attach files to emails - I start a new message - drag or attach a file - not big and the file sits in the email with 'downloading' written underneath. When I click on the file I get the message " ERROR - Mail was unable to save the attachment “K470923.pdf” to disk. Verify that your downloads folder exists and is writable." The file is on my desktop - I'm not trying to download it - iMac 27" 2022 - OS is Sequoia 15.6 128GB ram - 100GB free space - I have reinstalled my system to no avail! Please don't make me use Outlook.