In Junos Space, CLI Configlets are used to apply configuration. They cannot be used to execute commands such as "request system storage".
If you wish to use Junos Space to execute commands, then you can leverage SLAX scripts. It used to be possible to have SLAX scripts locally executed by Junos Space without the need to deploy the scripts to the target device(s). Not sure if that feature is still supported or not on Junos Space though, since I've not done anything for quite a few releases now with Space.
But if you wish to perform operations such as storage cleanup, snapshots, RSI etc, then SLAX would be the approach I'd look at with regards to using Junos Space.
Regards,