"Why doesn't my multiplayer work? I have two separate systems and two accounts on LAN, and I want to test Steam multiplayer.

Join and Create

In player to create a session and find and join one

I am using two computers on the same LAN with two different Steam accounts. When I try to click ‘Join Game’, it loads for a moment but then kicks me back to the main menu or starting level. I know that something is crashing, but I don’t know why or where to look. I need help to fix this multiplayer issue.