Running Sierra and Leopard on MacBook Air
If the machine is newer than Leopard, then it's not possible.

No Mac can run an OS older than itself - the drivers for it simply weren't invented.

Your only option would be to set up a VM, using Parallels or similar; but you'd have to use Lion - the oldest user version of OS X licensed for virtualisation.
Snow Leopard Server was the first ever version you can virtualise.