The lobby host and any players wanting to join must have the exact same mods installed to be able to play. So likely either you have some mods installed or the host does. A lot of people deleted their mods folder thinking that would clear up the issue, but you also need to check the mods tab in-game and hit the "Collection" button on the bottom right to see if you have any subscribed in there.

I don't have any mods installed only maps. But I installed the version of the previews patch 1.2 and I went back to 1.1 the multiplayer worked and now with 1.2 ofiicial I didn't have
access to any network part.
I checked the files with steam 0 error. I think there is still a file from previews 1.2 which blocks the network games.
Je pense qu'il reste un fichier du patch 1.2 previews qui m'empeche dans le multiplayer
How to do it Thank you

i supp also maps in collection now is good
Thank