I ran these commands:
juju generate-config juju switch manual
Then I edited the environment file to add the host address.
juju bootstrap juju deploy --to 0 juju-gui
And I got the error message:
ERROR cannot assign unit "juju-gui/0" to machine 0: series does not match
I got similar errors trying to deploy mysql to another machine after add-machine but I guess the answer to the above question can solve the other case as well.