Let's say your object that your spawning is named, "Enemy". And your trying to spawn him every 5 seconds. I know that's not what your doing, but you didn't provide enough details.
So try putting this Event,
Every 5 seconds > System create object, Enemy.
Put what layer you want the Enemy on.
For the coordinates of spawning use, X:random(width of your layout)
Y:random(height of your layout)
So Every 5 Seconds a "Enemy" spawns at a random spot anywhere on the layout.
-But your question is what if he spawns in a block or somewhere he can't move?
To solve that, use this.
Enemy is overlapping Platform>Destroy Enemy
>Create Object "Enemy" on layer 0 (random(width of your layout)) (random(height of your layout))
Basically when he's created and he is somewhere he is not supposed to be, he will be destroyed and placed in a new spot, if this keeps happening he will keep spawning until there is somewhere "spawn able."
This is a bad example but it can be worked with, if it is not fitting your game tweak it the way you want it or just not use it.