This is a endurance mission on Pluto.
You need to have Pluto unlocked or know someone with Pluto unlocked if that wasn't already obvious.
Make sure to come well prepared as this can be hard even with a full group of four.
The in-game chat and matchmaking has been down most if not the entire time this third phase of the event has been going on.
So you'll need to either:
-Team up with people that you're already friends with
-Find someone on a forum
-Or try finding someone that is twitch streaming
You can still send invites to people, you just can't have the game match you up with randoms.