Here, $O$ is the center of mass of a uniform, rigid and solid block. $F_g$ is the force of gravity acting on the body vertically downward, and $N$ is the normal force acting on the body vertically upward. The coefficient of friction of the floor is $\mu$ (distinguishing between $\mu_s$ and $\mu_k$ is probably unnecessary here). So, if the body is pushed horizontally, then the frictional force will be $F_f=\mu N$.
Here, $O$ is the center of mass, and we are in space, so no force of gravity exists. The coefficient of friction of the floor is $\mu$. As no normal force exists in this scenario, the frictional force is zero.
We can evidently see the two scenarios are different. One has friction, the other doesn't, but why is this the case. The net force on the block in both cases is zero, so why does friction exist in scenario 1 and doesn't exist in scenario 2? Aren't scenarios 1 and 2 essentially the same as the net force on the block is zero?
- Why does friction exist in scenario 1, but doesn't exist in scenario 2?