This article details the steps required to create teams meetings from within Outlook 365. Some users are experiencing issues once they have been upgraded to Outlook 365.
Meetings will only work from within your own calendar. Simply having other calendars open can cause the creating of teams meetings to fail.
Before attempting to create a new Teams meeting, ensure all you have 'selected' the correct calendar by making it the active one first. Once you have clicked the "Calendar" title, you can then click the Teams Meeting button to create a new item and invite attendees.
Meetings created here wil sync to the calendar view in Teams as well.