Edit - Well my bad, for some reason I assumed you were talking about your own MP server. You CAN do that and play by yourself with all the maps if you like:
Hey man i can help you with the maps - hit me up on steam: http://steamcommunity.com/profiles/76561198065341973/
Basically, once you've subscribed to the maps in the workshop they have all downloaded and can be played in SP but the server cannot access them because it is actually pulling the maps from this folder:
Server: C:\Program Files (x86)\Steam\SteamApps\common\Project Zomboid Server\media\maps
Client: C:\Program Files (x86)\Steam\SteamApps\common\ProjectZomboid\media\maps
For whatever reason the maps are not integrated into the multiplayer aspect of the game from the workshop. Don't quote me, but they won't load if you don't install them to the main folder (the actual PZ folder I showed you). They download here and have a corresponding number:
C:\Program Files (x86)\Steam\SteamApps\workshop\content\108600
You will go in there an find each folder that contains the actual map data, like this:
C:\Program Files (x86)\Steam\SteamApps\workshop\content\108600\532252577\mods\New Denver, BC\media\maps\NewDenver
So - for New Denver, BC the folder containing the actual map data is called NewDenver. Your server needs to access THIS - but it is trying to access it from the main folder (C:\Program Files (x86)\Steam\SteamApps\common\ProjectZomboid\media\maps)
Take all of these individual folders and place them where the server is attempting to access (COPY and PASTE, rather). (C:\Program Files (x86)\Steam\SteamApps\common\ProjectZomboid\media\maps) Once you've done that, edit your servertest.ini file. Here is mine:
Mods=Hydrocraft;AdditionalHairColors;Antize;PumpPower;TheWalkingDeadPrisonV1.3;Bedford Falls;New Denver, BC;Rebuild
Map=TheWalkingDeadPrison;Rebuild;NewDenver;BedfordFalls;Muldraugh, KY
Be sure to list Muldraugh, KY last so that the mods can take precedence. (The Mods line will contain the name of the folder that contains the map folder, see example above.)
And further down:
WorkshopItems=524066677;532252577;500222958;522891356
Those numbers are the numbers which correspond with each map's workshop ID. You only need to put the map ID numbers here. The mods simply need to be listed where I showed you in my example. You may or may not need to copy any mods you want to this folder:
C:\Users\(your username)\Zomboid\mods
I did it and we are running smoothly so I've left them there - but I didn't have a problem with the mods so I haven't looked into it. Any questions feel free to hit me up on Steam! This should help Happy zombie hunting, survivor!