Hi, Outpost Rush requires to have a level 60 character. Can you confirm that you can’t sign up although you have a level 60 character? In your screenshot, I can see that it says that you have a pending group invite, have you tried to cancel that group invite and sign up again?
Also there is a minimum of players required for the game mode to launch. It needs 32 players in the queue. When you sign up, a banner will appear where you will see the number of players currently in the queue. If it’s below 30, the game won’t launch.
Can you check and let me know if that still doesn’t work?