Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Be patient!
I've just found the feature is currently in limited preview to a subset of subscribers.
Details you may find at https://support.office.com/en-us/article/get-a-personalized-email-address-in-office-365-75416a58-b225-4c02-8c07-8979403b427b?ui=en-US&rs=en-US&ad=US
It seems that this is why you haven't still get access to the feature.