"Bullet on collision with Enemies" picks an instance of Enemies family. But the BulletHitZombie function doesn't know about that. So in this case you need to pass the UID of the of the picked to the function. Create a parameter enemyUID, pass it to the function, and pick an Enemies instance inside the function by UID.
Another option is to enable "Copy picked" in function properties, then it will automatically pick instances from that main event.
And the third option is simply move all those actions from the function into the main event - "Bullet on collision with Enemies".
But, this is important: since your events are picking Enemies family instance, all actions should also apply to Enemies family - not to the Sprite as on your screenshot!
EDIT: Right, there is a 4th option - use a custom action, it will work very similar to a function with "copy picked".