Unity reserves special folder names that change the context of the assets contained within them.
One of these folder names is Editor.
If your script is under a folder named Editor—anywhere above it inside the Assets directory—then it is inside of an Editor-only context.
Having runtime scripts in an editor context is invalid.
Move the script outside of the context created by the Editor folder. If the folder isn't intended to be Editor-only, you should rename it.
If you are using a scriptable object or prefab, this should also not be inside an editor context.
Unity, huh, how?