Why does it only show a black image when streaming?

I wanted to stream on Twitch with Streamlabs OBS. I started the stream and got into the game. Then on my mobile I wanted to watch my own stream on twitch but it only showed a black picture although I was in the game… The audio went well and was normal only the screen was black… How do I get that normal?

Add a new source, click Game Capture / Game Capture, name the source, and then select Capture Specific Window and then the game.