I think maybe you installed snapcraft from edge, given your command line shows revision 2381, which is only in the edge channel. I just reproduced your steps and it works for me.
root@sensible-jaguar:~# snap install snapcraft --classic
snapcraft 3.0.1 from 'canonical' installed
root@sensible-jaguar:~# snapcraft
You need multipass installed to build snaps which use the base keyword.
Would you like to install it now? [y/N]: n
multipass is required to continue.