In case you are setting up your M365 and setting up Exchange Online in the process, your Email URL is: https://outlook.office365.com/mail/

But, you see the below error which reads as

err: Microsoft.Exchange.Clients.Owa2.Server.Core.OwaUserHasNoMailboxAndNoLicenseAssignedException


No License Exception

Here’s why you see the Exception.

1. If you open the User record in the Microsoft 365 Admin Portal, you’ll see that Microsoft Exchange Service is not seen in the list of Services.
   
   
2. Now, let’s procure a trial for Exchange Online or any of the E3 / E5 plans which you plan to purchase based on your requirements. They have Exchange Online services available which will enable your Exchange Online mailbox.
   In this example, I’ll start a trial because I don’t want to buy for Enterprise purposes for the sake of this example.
   
   
   
3. Now, once I started this trail. And if you see the comparison chart above, you could even opt for other licenses that offer Exchange Online – say, Office 365 E3
   Now, I can see that this license is available for me to assign to the user.
   
   
   
4. And when I select the above license, I will see the Service available in the list as well.
   
   
5. Save your licensing changes / preferences and let 5-10 mins pass for the services to be applied to the User.
   And once you reload/refresh the Outlook web app, your mailbox will be available or retained (if it was expired on an existing license.)
   
   

Hope this helps!