How to enable ssh on JunOS 19.4?

First of all, we need to set root password as default one is username root with no password.

Then run cli, then switch into configuration mode with: configure

And apply following command:

set system root-authentication plain-text-password

You will be asked to provide password and password confirmation.

Then you need to apply change using commit command. 

After that enable ssh:

set system services ssh

And enable root login over ssh (not recommended for production use):

set system service ssh root-login allow 

And finally apply all changes using commit command.