Weird message when I hit Send/recieve on Outlook on Mac
You likely have an email stuck in the Outbox.

Outlook is nowhere near the horror show it used to be but it is a complicated beast that wouldn't know an understandable error message if it bit it.

So look down the left hand side of your Outlook window in the folders. There is likely one there called Outbox and it is in bold possibly with a "(1)" next to it.

Open the outbox and the email in it and verify that the email address is correct and any attachments are not too big for your mail service and click on the send button.

Failing that you could copy the email into a NEW message and try sending it again.

If you are otherwise sending and receiving email then you are not in offline mode and it is most likely it is an email stuck in the Outbox.