My friend and I may be having issues, I’m afraid.
We just tried playing some Private games, both via Custom Lobbies and Exhibition matches, and we were never able to join together.
We’re both playing on PC, the Windows 10 version. Each of us have passed the game’s Performance Test, and we checked our connections via Start Menu>Settings> Gaming>Xbox Networking, and our NATs are open and we’re both connected.
Whenever we would accept an invite from the other, it would try for several seconds, and then display the error message: “Couldn’t Start Multiplayer. Unable to start multiplayer match. Please try again.”
We attempted to recreate Lobby’s and such, and join, several times.
We have just been playing Mortal Kombat 11 (PC) for about an hour without issue. This is obviously over Steam, and it’s a completely different ecosystem, simply pointing that out to show that we can play together.
If there’s any more info you need, please let me know!