I finished a map yesterday, and was going through and was attempting to put in the game types, like CTF, KOTH etc. For some reason I get this message "This map variant is not compatible with this game variant" Am I missing something simple? Edit: I don't know if I'm going into detail enough, but its not allowing me to edit those gametypes at all.
I've heard of other people having this problem, apparently there are certain objectives you MUST have in order for the map to support gametypes IE: A map must have hills placed to support KOTH Most likely, yes you are missing something simple, just check all of the objectives and see if their remaining left to place are less than the total for everything that is related to the gametypes you want to be on it.
I suggest going into forge and making sure that everything is setup correctly by running through all of the gametypes
Your map is missing required objects. I don't know what gametype you're using but read this, it should solve your problem. Bungie.net : Halo: Reach Forum : Forge Required Objects Guide