the passdb backend entry in man smb.conf say that "Available backends can
include:". It should be "Available backends are:" and a _complete_ list of the
backends. plaintext backend is missing there.
Not all backends are available at all times (and additional, 3rd party ones
can be added using plugins). That's why the sentence used is:
"Available backends can include:". There is no 'plaintext' backend.
If you're talking about the /etc/samba/smbpasswd file, that backend is called