Hello, I’m using the steam online subsystem with advanced sessions plugin, When the player 1 creates a session and p2 tries to join, it is a success but nothing really happens.
It should load the player 2 to a level where p1 is in Open Level is set to listen
Looks like you’re telling the joining player to open a level.
On connect the server will tell the client what level to open. You don’t have to do anything here.
It’s actually connected to a host session so I think it has to know what Level to open when creating the session
IF you’re doing P2P (listen) then the Host should be pre-selecting the map, because you are the server. When you as the server are ready, then you the server is advertised. “Joining” clients do not explicitly call Load level. The game will handle that.
IF you’re doing dedicated, then the server (non-client hosted) should be running whatever map. Any client joining will be told what to load.
It is like you said, when someone starts the session, it loads the level and then players who join should load the level the host is in but it doesn’t happen, every tutorial I saw does the same thing I do and it seems to work fine for them… 
I created the whole system from the start again and it doesn’t seem to be a problem anymore
