Check your objectives. Not all the maps support all the objectives—or in the numbers you may have them. I just dealt with this recently that some of the outdoor maps didn’t support all the things I had in the mission, so that narrowed the choices down quite a bit.
If the Publish button is grey, then something is generating an error, even if there are no errors called out. Try making a mission with one simple objective clearly listed in the map’s allowed objectives. If that mission doesn’t allow publishing, then try with different enemies. Sounds like there is some kind of conflict somewhere.
Sorry can’t be more specific without analyzing the whole of your mission settings. 
Try to reduce the quantity of your spawns by a significant amount. See if that solves it. You can then incrementally increase it (or actually check the map to see what the maximum allowable is)
I know certain maps have been modified to reduce their maximum farmability. I don't use city02, so I don't know if it was affected, but it was a popular farm map.
I can't figure this out.
I've quit and restarted after making all the changes I wanted, the errors display tells me that there are no errors. I am trying to make a map using Unique maps -> Outdoor maps -> city map 02. Its part of the unlockable outdoor map content.
When I switch to any other map big enough that there are no errors, it publishes just fine. When I put it on citymap 02 the publish button greys out and so does the test button, as if I cannot access that map. I don't get it.
help would be appreciated.