Using New Outlook on Windows for professional communication and productivity
It seems that those who have the "Exchange Online (Plan 1)" license cannot use the New Outlook, because this plan only gives the right to use applications in the browser, and the old outlook does not validate this license, so we were able to use the old one.
To check what plan you have, go to Microsoft web "My Account" > "Office Apps" > "Subscriptions" and look for "Exchange Online".
People keep coming to this conclusion, but look at my previous response (end of page 3), it for sure is not strictly based on licensing. My post goes over how 1) paying for higher tier licensing did not resolve the issue [by upgrading licensing for the account in question], 2) I have accounts on Exchange Online 1 licensing which DO work in the new app.
It does not appear to be exclusive to licensing.