if you are using Hybrid mode of office 365 with exchange on premises scenario, you will likely to be encountered with the subjected error while creating a meeting for MS Teams from Outlook both for MAC and windows. This article is the Hack to get this issue resolved as this is the bottle neck for Meeting as adding Teams Ad IN in outlook is also bit tricky some times. Follow the below steps for subjected issue.
1.Quit all Office applications in your Mac device.
2.Open Microsoft word, sign out and sign in back with your office 365 Account.
3.Intall MS Teams Desktop App and sign with same Account.
4.Lauch outlook and in settings make sure that user name should be in Syntax as demonstrated below. Domain Name is your Active Directory Directory FQDN.
5.Once done Just try to create Meeting, Error should be gone.