You also need to add the explosion scene into the scene tree. If you're calling this on the enemy that is being destroyed, try adding it with add_sibling(explosion_instance)
Whatever you do, do not add it as a child of the enemy scene, as you are probably removing the enemy scene immediately after, which will also remove all its children.
#Playing Explosion Node 3D
odd walrus
vale seal
Ah, that makes sense.