This weekend I needed to use Fusion for the first time since I upgraded to macOS Mojave. Having run Fusion 8 for many years and being happy with it, I was somewhat annoyed with needing to upgrade to Fusion 11. At least that appeared to be the consensus on the interwebs. In retrospect, I’m not 100% sure. That said, I’ve received great value from Fusion, and I don’t mind paying money for good software.
Unfortunately, after upgrading I received a similar error as before with Fusion 8: “Cannot open /dev/vmmon: No such file or directory”. After spending a fair bit of time researching this error, the verdict on the VMWare forum appears to say that you cannot have VMWare Fusion and VirtualBox install simultaneously and that you need to uninstall VirtualBox. Since I occasionally use VirtualBox (through Vagrant), that felt like a sub-bar solution. Fortunately, I found this blog post that outlines how to simply unload/load the VirtualBox kernel modules on demand instead.
However, after doing this, my error still remained. As it turns out, I have missed some kind of System Preferences dialogue where I allowed Fusion to load its kernel modules. Details can be found here. Hence, if your error still remains, make sure that you double check that. Doing this would have saved me time.