I have the same issue and was able to use the new Outlook until recently with new computers I set up. Currently it works on the existing one I set up in October 2023 but does not work now on any more computers so they must have changed this.
Also if you are coming from the Windows Mail client then after signing in it prompts you to switch to the new Outlook client which if you say yes to you then have no working email client which is far from ideal. The same happens if you are using Windows Mail and press to try the new Outlook which leaves you without any Outlook so this process has not been very well thought through.
Does this mean Microsoft are removing the free email client from Windows and offering only one you must pay for now?
Also if you have purchased Office 365 as a standalone product you are also stuck without a working email client if you switch as it seems it requires the licence to be tied to the Office 365 account.
This seems a mess.
Also whilst I like the new email client overall it has some significant usability bugs such as the right-click menu that remains on the screen obscuring the view and after you have a few in the way you must restart the application.