The 'Unity cannot restructure prefab instance' error typically arises when there are attempts to modify a prefab in an unconventional manner, specifically when trying to change or remove game objects inside the prefab instance in the scene hierarchy. This could be due to changes made either through scripting or in the Unity editor.
Prefabs are meant to be blueprints for game objects that can be reused throughout your game. When you create an instance of a prefab in the scene, Unity maintains a link between the instance and the original prefab. This is so any changes made to the original prefab can be easily propagated to all its instances. However, if you try to heavily modify an instance of the prefab (such as restructuring its hierarchy), Unity might struggle to maintain this link and throw the error.
To resolve the error, bear in mind the purpose of prefabs and use them as intended. If you need different behavior or structure in certain situations, consider the following solutions:
Variants: Use prefab variants if you want similar but slightly different versions of the same prefab. Variants allow you to override properties while still maintaining a link to the base prefab.
Nesting Prefabs: If you want to restructure a complex prefab, consider breaking it down into smaller prefabs and nesting them. This way, you can make changes to individual parts without affecting the overall structure.
Scripting Changes: If you're modifying the prefab instance via script at runtime, ensure these modifications don't conflict with the existing prefab structure. You may need to adjust your code so it respects the base prefab's structure.
Unpack Prefab: If you need significant customization for a particular instance, you can opt to unpack the prefab. Keep in mind, however, that this will sever the link between the instance and the original prefab, so changes made to the prefab won't affect this instance anymore.