Steps:
1. Create a prefab out of any GameObject
2. Open it in Prefab Mode
3. Select root GameObject
4. Click on "Add Component" button and choose New Script
5. Type a name for your script
6. Click on "Create and add" button
==> "You are trying to replace or create a Prefab from the instance 'Cube' that contains the script '<ScriptName>', which does not derive from MonoBehavior. This is not allowed."

Note: Script is created but not added to prefab root GameObject