Hi Roni,
From your description, I would like to confirm the following things:
1. Where did you get the error message? Please capture a screenshot.
Note: To insert a screenshot, please click on Use rich formatting and select the Insert Image icon.
2. Is this the first time you activate Outlook for Mac? If so, please check if you have assigned Office 365 ProPlus license.
3. Are other user in your organization also experiencing this issue?
Best Regards,
Vincent Wu