$ juju destroy-model default
WARNING! This command will destroy the "default" model.
This includes all machines, applications, data and other resources.

Continue [y/N]? y
ERROR failed to destroy model "default"
ERROR cannot destroy model: failed to destroy model: manually provisioned machines must first be destroyed with `juju destroy-machine 4`


$ juju destroy-machine 4
ERROR unrecognized command: juju destroy-machine