Request Highscore is used for loading data into the game.
Display Highscore is used to show the leaderboard in an external window in the google play gui.
So those actions are not connected, so you are using the plugin wrongly.
Just try to have a button that uses Display leaderboard, nothing else. and verify that it works first.
(Last time I used google play, the condition Is logged in, didn't fire. So try to avoid it, and see if it works)