Hi there, I´m Ivania. I´m an Independent Advisor and a Microsoft user like you. I´ll be glad to help you today.
Please try clear the folder cache by right clicking the folder and then click Properties > Empty cache in Outlook for Mac.
Some users managed to get rid of this message by cleaning both the inbox and the outbox.
If it doesn't work, try to delete and re-add your account, it could be that the account is corrupted.
Best regards,
Ivania