Can you be more explicit about your inability to delete messages in TB? The email account is it working with the POP or the IMAP protocol? How do you try do delete emails in TB? Clic on one of the "Delete" buttons? Using the contextual menu? Using a keyboard short cut or a delete key? Are all messages in all folders of all accounts concerned? Do you see any error message issued by Thunderbird or the system? Did you try to delete messages after (re-) starting TB in troubleshoot mode? https://mzl.la/3AbU5jz

Thank you for your comment 1. I cannot delete messages , by any means (pressing the delete button, pressing the trashcan image, or even by moving the message to the trash folder). 2. I am using a POP account 3. I see this problem mainly in my INBOX folder 4. NO system or TB errors 5. How can I start TB ( in the MAC) in troubleshoot mode ?

Thunderbird troubleshoot mode for macOS: TB already running go to Help > Troubleshoot mode .... TB closed: hold the Alt (Option) key and launch TB....

One more question: do you store your Thunderbird profile on an UNC drive?