I encountered the same issue but if anyone is having issue with the inefficient permission when adding a video to a playlist, you will need to have the YouTubeService.Scope.Youtube
(which looks like you already have).
var scopes = new[]
{
YouTubeService.Scope.Youtube
};
If you however added the scope after you already have given the permission, you will need to revoke the client by going to this page manage permissions. You will have to look for your specific client. After you've done that, you can rerun you app and request for permission once again.
Another option is to create a new clientId and clientSecret again and make sure you have the right scope to begin with. I hope this helps.