Assign the camera to a variable in a prefab

You can't assign a object from the scene to a prefab in memory. What would happen if the scene changes? What are you trying to do?

Prefabs are Templates - they can only reference other Prefabs. Imagine you could reference Objects in the scene, but then dragging the Prefab into another Scene. Objects in the scene can reference Prefabs, but not the other way round.

What exactly do you want to do?

Is the variable in the script of type Camera? If you only have one Camera and it's tagged "$$anonymous$$ain Camera" (the default) you could do:

 private Camera mainCam;

and in Start() do

 mainCam = Camera.main

or FindObjectWithTag("$$anonymous$$ain Camera")

(untested).