I've seen some people facing that problem. Well, my friend had it as well. Downloading it via the link also doesn't do anything cus you'll get a critical error.

So, to fix the "can't see the map" problem you'll need someone who already has this map. Just ask them to send you the map's .ROM files via Discord, google drive etc. Those can be found here:

Steam library ---> killing floor ---> properties ---> local files ---> browse local files ---> Maps. The person that has these maps will have the following 3 .ROM files there:
KF-TestZSpawningZoneV15.rom
KF-TestZSpawningZoneV30ULTIMATE.rom
KF-TestZSpawningZoneV30ULTIMATE[SOLO].rom

After you get any of these("any" cus each one is a different map) you just put them in your Maps folder. Done, now you should see these maps in game. At least I think so, because this worked for my friend.
(Yeah, I guess that's workaround, not a fix :p)