I remember encountering that exact same error too but I do not remember exactly what I did to fix it.
(The below is for "Game Variants > Attributes Tab")
Might be an issue with the "Team" settings. Make sure the "[#]" in Team 1 [1] Team 1 [2] (for example) all have different numbers because they represent the placement slot on that team.
It should go Team 1 [1], Team 1 [2] (for example) and not Team 1 [1], Team 1 [1].
What I use is "Games Teams [Premade]" (Leaving "Games Teams" empty and unedited) and edited the "Player Team [Premade]" settings.
Though you can use the non-premade version too (if you get it to work) but when I did it, it did not set up the lobby correctly for some reason even though I checked all settings.
If that doesn't work, try changing some of the other settings (try experimenting with some settings and see if it fixes the problem, sadly I did not remember what I did to fix the problem when I encountered it).
Also remember to make sure that the "Map > Player Properties" settings do not conflict with the "Game Variant > Attribute" settings [they should be the same if possible with the exception of the "Races" field which should be set to (Any)].