Stomping is a bit fiddly.
1) both feet must be on the ground when you start (no changing of animation mid step)
2) the target needs to be close enough to to the rest position of one of the feet so it can be reached in one step without stretching much. (target is whatever the camera is pointing at when you press space, not counting the mech itself)

In walker.lua you can uncomment this line to see how big the area is
StompAreaDebug(Right)

every time a press Space it wont let me stomp

same