Error unable to join a session. Please check your network connection and try again

Press Win+R and type firewall.cpl and click ok. Click advanced settings on the left, and when the new window opens, click Inbound rules on the top left.

Here is the list of  rules, search for one named Plants vs Zombies Garden Warfare™(Should be 2, do this for both) , right click it and choose properties. In the properties click the advanced tab, check the public and private box and click apply.

Alternative is just to delete the two firewall rules, and start the game. Firewall window will ask you if you allow this program, and you click the private and public networks.

BTW, I'm still having this problem sometimes, but it seems it depends on the host  😕