OK, I love Veracrypt, it is a very nice tool for running encrypted volumes on your system. On the Mac though, it uses MacFuse underneath and you can run into not having enough file system slots to run it. You will get a cryptic message like:
mount_osxfuse: File System is Not Available (255)
Thank goodness for the internet, but OSX Fuse explains that there are limited slots and that if your have VirtualBox and OSX Fuse running on macOS Sierra, you can run into trouble.
On High Sierra things are bit a worse, so the steps are to look at what kext you have loaded with
kextstat | grep -v com.apple which looks for non Apple kernel extensions and see things that could be about the file system.
In this case I had to unload VirtualBox since we use VMware Fusion, but I needed one more slot and saw that sometime I had loaded ExpressVPN and it had
expressvpn.tun taking up a slot, so you run
sudo kextunload -b expressvpn.tun and it loads.
ExpressVPN cannot just be deleted it turns out, you have to run an uninstall program (See https://www.expressvpn.com/support/vpn-setup/app-for-mac-os-x/#uninstall)