The underlying cause for this is that vboxsf does not support filesystem links (neither hard nor symbolic). You can reproduce the issue by mounting a host folder inside a VirtualBox guest and trying to do the following:
ln /path/to/mounted/share/existingfile.txt /some/path/link-to-file.txt
for example (with /var/data mounted as a vboxsf share)
jim@a55a005fc5a5:/~ ln /var/data/foo.txt ~/data/link-to-foo
ln: failed to create hard link ‘/users/jim/data/link-to-foo’ => ‘/var/data/foo.txt’: Operation not permitted
There are numerous tickets for this at virtualbox.org (#818, #10085) but seeing as they go back 7 years, it might be another while yet before it's fixed!