VMWare Fusion: Could not open /dev/vmmon error
After having this issue with macOS High Sierra and VMWare Fusion 8.5.8, I was able to resolve it by allowing the system extension as described in http://planetvm.net/blog/?p=3240

In short; open System Preferences, click Security &amp; Privacy, and from the General tab you should see a message stating System software from developer "VMware, Inc." was blocked from loading. Clicking the Allow button resolved the issue for me.