I have stadia pro and i can play all my paid games and pro games but when i click on hitman stadia just darkens and nothing happens.
And this is playing on mobile? Since you posted it there.
This sound very strange, especially if other games work. Are you playing with a controller or with integrated screen controls?
I'm playing on my PC with mouse and keyboard and it worked before
Then you posted in the wrong section of the forum.
Have you tried waiting when starting the game?