In the most recent version of win 10 it adds what it calls "exploit protection". This is aimed to protect users against.. malicious software and other stuff. However the mod uses stuff to load its contents into the Just Cause 3 Binary so that it has all the multiplayer. And this protection doesn't allow that. So we have to turn off that protection :/

Tl;dr:

1. Go into you're windows defender
   
2. Click App & Browser control
   
3. Then scroll to the bottom and click exploit protection settings
   
4. Click the tab that says Program settings
   
5. Click the +"Add Program to customize"
   
6. Click "Chose exact file location"
   
7. Go to the path "C:\Program Files (x86)\Steam\steamapps\common\Just Cause 3 Multiplayer Mod" (Depending on where your steam library is.. that is just the default)
   
8. Click PlayJCMP.exe
   
9. A window will pop open, scroll down to "Force randomization for images (Mandatory ASLR)"
   
10. Click override system settings, and make sure the toggle in that subheading says "off"
    
11. Click apply
    
12. Try to start the game

If that doesn't fix it.. Try running the game as a administrator.. other wise ¯\_(ツ)_/¯

Pics[imgur.com]