Can be for several reasons:
-- Windows --
1. The Map File is Corrupt or Unreadable
2. You're running with a Sandbox Tool like Sandboxie or Wine (for linux/mac users), which can mess with files and make it impossible to open the map.
3. Your antivirus could be conflicting with the Launcher/Map Contents.
4. You may have low space.
5. You may need to update drivers.
6. Novetus/the Map didn't download correctly, causing the first reason.
7. The Map could be Unconverted. Try converting the map with a guide from the Finobe Wiki, or 1.2's Converted Tool and try again (if its a map you downloaded. Useless with maps you're trying to run that came with Novetus, because they're already converted.)
-- Linux/Mac Through an Emulation Software --
I know nothing about Mac and Linux OSes due to my lack of knowledge. I've never touched Mac OS X. I touched a few Linux distros once but then I stopped. The way Wine works is simular to the effects of a Sandboxing tool, and that may lead to reason 2.