The message “email_subject” could not be moved to the mailbox “(null)”

Using macOS 10.14.6, on a MacBook Pro (Retina, 15-inch, Mid 2015), Mail Version 12.4

I highlight an email in my inbox. When I attempt to move the email to junk by clicking on the junk icon in the menu bar at the top, I get the message in the above subject line. So currently junk mail is filling up my inbox.

I quit out of mail and then reboot the MacBook. Bring mail back up. No change.

I click on the junk mailbox on the left-hand column under 'Mailboxes'. Then I click Mailbox > Rebuild.

Nothing seems to happen and the above behavior does not change.

Does anyone have any suggestions as to how I can get rid of all these junk mail items?

Thanks!!