It sounds like you may have a non-core version of Flexmailer in the mix (like Flexmailer 1.1.1 instead of Flexmailer 5.65). See this past question for instructions to remove.
Per the previous question, the steps are disable Mosaico, then Flexmailer. Delete the Flexmailer directory. Clear the caches. Enable the new Flexmailer, then Mosaico. Perform database upgrades if a message tells you to.