Skype Takes Over as Default Online Meeting in Outlook (MAC)

Login to https://portal.office.com and go to Outlook and then the Settings Gear (top right)

From there you will need to set Add online meeting to all meetings and then select Microsoft Teams. 

Then reboot the MAC.

https://answers.microsoft.com/en-us/outlook_com/forum/all/how-to-replace-skype-with-teams-as-a-default/31e4b2fc-f629-40ab-aece-0668f2e2c9d1