Why am I receiving this error when trying to compile?
Saving Prefab to immutable folder is not allowed: Packages/com.unity.render-pipelines.core/Runtime/Debugging/Prefabs/Resources/DebugUICanvas.prefab
UnityEditor.BuildPlayerWindow:BuildPlayerAndRun ()
I tried deleting the Library folder, and that didn’t work.
Any help is welcome!
You can not and should not mutate packages, nor should packages be mutating themselves.
spiney199 that doesn’t help… i didn’t do this… it did it itself… now… how do i fix this?
Well it does help. You know now that packages shouldn’t mutate themselves. If this is with a Unity package, see if it happens in a fresh project. If so, report a bug.
Hey AerionXL! (Or anyone else that might be seeing this, with a sudden pop up!)
Theres a few bugs that can cause this however they are rare.
For me, it happened while trying to APPLY to an already made Prefab (updating my prefab as i was working outside of the prefab viewer)
Open your prefab, and check for components or scripts that are missing inside the SAVED prefab. you must remove this before you can apply your changes! Don’t worry, removing a component in the prefab editor will not change the existing version of the prefab you are working on in the scene hierarchy!
Hope this helps someone 
P.S.
my immutable problem was caused by deleting a script and not removing the component from the ALREADY created prefab!
For me it is happening with GoogleAdmob Plugin, which is not my prefab.
Saving Prefab to immutable folder is not allowed: Packages/com.google.ads.mobile/GoogleMobileAds/Editor/Resources/PlaceholderAds/Banners/BANNER.prefab
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)
Saving Prefab to immutable folder is not allowed: Packages/com.google.ads.mobile/GoogleMobileAds/Editor/Resources/PlaceholderAds/Interstitials/768x1024.prefab
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)
I have the same issue… Any solution to this?