Question

there is no air on surface of moon, so how do astronauts communicate

Answer 1

they communicate by radios in their suits.

Answer 2

The space is actually a vacuum where there is no air.  The sound travels through a medium every time may it be a solid, liquid or air/gas. On the surface of the moon there is no medium for the sound to travel. Hence " they use radio which produces the radio active waves " to communicate on the surface of the moon.