if you're trying to join and the game tells you the map is missing or differs from the one you have, you need to go to steamapps and see if you can find it in your map directory. At least, this fixes it for me.
Find your steam folder on your system, and go to D:\Steam\steamapps\common\Team Fortress 2\tf\download\maps
From here, find the specific map in question that you are getting the error for. If its here, delete it, this will force the game to redownload it as you join and hopefully will correct the issue