Hi @Dharma Liyanage 

Just check it once.

Note : "Start Meeting" button is only visible to the designated CAB Manager or a user with the admin role. 

https://www.youtube.com/watch?v=5j_NKRWJRcM

Also for MS Team Meeting option , just check once whether this plugin is installed (Ref: KB2102584 MS Teams Meeting Extension not working 

Meeting Extensions for Microsoft Teams 
App id: sn_now_teams_ext