Using Classic Outlook on Windows for personal email, calendar, and contact management
I have the same issue. The only option it allows is to set or clear the flag. No dates. The ribbon customisation does not work either as the icons like "Flag: Due date" come into the ribbon as inactive/greyed out.
There is a workaround. Click and hold on the email and drag it over the "Tasks" icon in the side-bar. That will create a new task with the email in the contents. You can then set start and end dates, priorities etc as per any normal 'task'.
Outlook developers need to re-assess their decision to remove the old functionality. I believe it was a poor decision.