NFS server support in OS X 10.15.x Catalina?
Do you have the NFS client utilities installed on the Linux system? The error message from your Linux system states that /sbin/mount.nfs isn't present, which suggests that the NFS client utilities are missing. On Debian/Ubuntu systems they're in package nfs-common - install that package and see if the mount then succeeds.
If you're all you're interested in is sharing a directory between your VirtualBox VM and the host there's no need to use NFS. If you install the VirtualBox guest additions you can share directories quite easily between host and guest.