I am running Virtualbox 5.1.34 - I have many VMs where mounting shared folder is not a problem. My host machine is a very-capable Dell workstation running Centos 7.
I created a VM from ubuntu-18.04.3-desktop-amd64.iso, the install went fine, I mounted the proper GuestAdditions ISO and installed it, it appeared to be successful.
I have created the shared folder in the Virtualbox control panel and corresponding mount point folder on the VM, both named "v-host" (I've done this many-many times) I shut down and restarted the VM.
I was running:
sudo mount -t vboxsf v-host ~/v-host/
and it returns:
/sbin/mount.vboxsf: mounting failed withe error: No such device or address
I have three Ubuntu 18.04-server VMs (no GUI) that do not have this problem - I installed GuestAdditions, rebooted them and mounting the shared folder works fine.
I also have an Ubuntu 16.04 desktop where shared folder mount works as well. But I can't get 18.04 to work.