Update: The issue was caused by a server disconnection that we occurred earlier while being in Warzone. I just got notified that my friends got disconnected in the same way while playing DMZ. We started a party and everything seems fine. Warzone 2.0 is back.
It seems that Infinity Ward and Raven Software messed up quite a bit. Apparently, players are having trouble partying up after the minor update that fixed the in-game social menu. Those who do not own the multiplayer component (the paid version of the game) cannot squad up and start a match even with those who own the game.
Each and every attempt leads to a message to either purchase Modern Warfare 2 or Install the Multiplayer.
There are a couple of issues right now. Those who own Modern Warfare 2 are being transferred to the Multiplayer lobby. While it allows them to search for a Warzone match, they cannot change their loadouts because they can only see the loadouts from the Multiplayer.
If paid edition players team up with free edition players, the free-to-play clients will get disconnected once you start the queue. There are already multiple outcries over on Reddit, stating that they are not able to play the game. Yes, it happens on all platforms.
While Warzone 2.0 is not working, players are able to squad up for a match of DMZ. At least my friends who do not own the game have successfully queued up for a DMZ match without being kicked from the lobby. We’ll have to wait until Raven Software, and Infinity Ward share further details. In the meantime, you can queue up for a match of DMZ while you wait for a fix.