In OBS, try "right click" in the OBS window (UI), then select add, then select "Display Capture". Then you can Start Recording.
Don't know what you are trying to record but if it's a game then you could also try adding "Game Capture" to your "Sources"
in lower LH corner of the OBS UI.
I am new to OBS myself, and still trying to figure it all out and at the moment I'm getting good video, but no audio ~:)
Ken