"This server is running a map you don't have."

This means your client was unable to find an installed map with the same name as the server's map.

One common cause is if the server is using a workshop map but you have that workshop file disabled. You can check your enabled/disabled workshop files from the menu here:

1. Open the Unturned main menu
2. Click Workshop
3. Click Manage Subscriptions
4. Any unchecked files are ignored when looking for a map

The game looks for maps in this order:

1. Is there a folder in the Unturned/Maps directory matching the server's map name?
2. Is there a folder in an enabled map-type workshop file matching the server's map name?

Behind the scenes

Ideally maps would have a unique ID so that the name wouldn't need to be unique. Copying maps to make a custom edit is very common though, so even if the copied map were renamed it would have the same ID as the original map. One feature that might help is if the server includes which workshop file the map is in so the client can differentiate two maps with the same name in different workshop files. (I intend to add this at some point.)