Any way to create a gamobject only if there is an empty space?

The simple way to do this would be to make an empty game object with a sphere/box collider about the size, shape and position of the object you want to spawn (I'm guessing you want something to spawn similar to forge mode in Halo), child it to your player, set "isTrigger" to true, and then have a script attached to it with the following:

 var canSpawn : boolean
 function OnTriggerEnter(other : Collider)
 {
      canSpawn = false;
 }
 
 function OnTriggerEnter(other : Collider)
 {
      canSpawn = true;
 }

Then just check if "canSpawn" is true when you want to spawn the object, then instantiate it.