Hello there,
Unfortunately the minimum supported Unity version is 2021.3.x. That is because Unity made quite a few changes between these versions and I had to make the code more managable. PrefabStage specifically is in a different namespace in Unity version 2020.2.x than it is in version 2021.3.x and this is the reason you are getting this error. I would suggest if possible to please update to Unity 2021.3.x or later, or you can of course also change the code to point to the correct namespace (PrefabStage was in the ‘UnityEditor.Experimental.SceneManagement’ namespace in Unity 2020.x if I recall correctly).
Gavalakis
