Co-op MP scenario - "unclaimed slots with locked civs"

Hi all -

I was trying to create an MP scenario where players 1-4 are human players against 4 AI. When trying to test out the scenario in a private lobby, I’m (1) not able to assign myself to anything except “player 1” (blue), and (2) I can’t launch the scenario as I get the prompt “A scenario has been selected for this lobby. Please select a player index for yourself as there are unclaimed slots with locked civilizations”.

Is this just not possible in the current game? It’s a bummer as the original game allowed this type of MP coop scenario. If this is not possible in the editor, how are the cooperative campaigns developed - and is it possible for us to create our own coop missions?