I am trying to join a game with my friend. She is playing a team deathmatch in Terminal. When I try to join I get the error that says This friend does not have a valid game session to join. What does this mean and how do we fix it?
It means the game she's playing in is full on both sides.
Team Deathmatch is 6 vs. 6. Unless someone from either side leaves the game, you cannot join.
Your friend can press Select and check the statuts of the game and see if there's a vacancy on either side, or you and your friend can start a Party. That way both of you can join games together as teammates.