Error: No such command "lets-encrypt"

I want to use Let’s Encrypt to setup HTTPS;
i follow the document of bench,but it have issue.
bench setup lets-encrypt site.local
Usage: bench setup [OPTIONS] COMMAND [ARGS]…

Error: No such command “lets-encrypt”.

The lets-encrypt command is in the develop branch of bench, it’s still in beta, so I wouldn’t recommend using it on a production site yet.