If sound does not travel on moon, it means it is a vaccum. but it's not true. Please explain.

The moon does have an atmosphere, but it's extremely thin, and sound does travel in space but you can't hear it.The density is about a billion times less than the Earth's, so in terms of conducting sound waves, it is pretty poor.

Sound is vibrations, doesn't have to be through air. If you had a helmet that conducted vibration, laid down on the surface, and had someone drop a heavy weight, you could hear the impact if the surface was hard enough to transmit it.