You are at the right place if you are looking for a complete guide about Fix the Co-op Multiplayer Not Working In Warhammer 40000. You will find a full guide about that problem with the most accurate and practical steps here.
Fix 1. Close and Relaunch Marine 2
Close Warhammer 40000: Space Marine 2 Game. Once the game is closed, wait for 1-2 minutes and then relaunch the game to check whether the problem is fixed. If not, restart your gaming device (PC, Xbox, or PlayStation) and then check space. Tip: Try again and again, and you may get past the error, and the problem will be solved. This might take several attempts.
Fix 2. Connect To a VPN
Now, this has resolved the problem for many gamers. I think this works for you as well. Reconnect to any reputed VPN and choose one located close to them, such as Chicago. Using a VPN to be “near” each other might be worth a try for playing with friends.
Fix 3. Turn off Crossplay
Re-Launch Warhammer 40000: Space Marine 2 and click to the Game Settings. And Turn off “Crossplay.” Now, check whether your problem is solved or not.
Fix 4. Turn Off Steam Overlay and Cloud Saves
Go to your Steam Library > right-click Warhammer 40000: Space Marine 2 > Properties > Turn off “Enable the Steam Overlay while in-game” and “Steam Cloud”. Now, launch the game and see if this helps.
I hope you have got the solution to your problem. If you have followed these steps, then your problem must have been solved.