The request body must contain the following parameter: 'client_assertion' or 'client_secret'

I am trying to get an authentication token for my app in order to push data through PowerBI API. When I run the c# script, I login to my MS account and my web app asks for permissions. After clicking accept, it returns the client assertion error. 

Why is this error popping up when I am registering a web app, not a native app. Is there any problem with the redirect URI? My homepage URL & Redirect URI are same.