Hi, there are several techniques for normal map baking in blender.
If you have used multires sculpting it is easy, set multiresolution back to 1 switch to multires normal baking, bake.
The other technique is retopoly, there you have to model a second object in lowpoly resolution.
You have to bake from the highpoly to the lowpoly object.
This is a lot more tricky, look for a good tutorial.
Often user have the objects on different layers, baking does not work then.
Opensuse Leap 42.3/64 i5-3570K 16 GB
GTX 760 4 GB Driver: 430.31
Octane 3.08 Blender Octane