Getting errors while creating a custom terrain editor

I created two scripts called "CustomTerrain" and "CustomTerrainEditor". Here is the code for the latter:

 using UnityEngine;
 using UnityEditor;
 using EditorGUITable;
 
 [CustomEditor(typeOf(CustomTerrain))] // Links editor code in here with CustomTerrain.cs
 [CanEditMultipleObjects]
 public class CustomTerrainEditor : Editor
 {
     void OnEnable() {}
 
     public override void OnInspectorGUI() {} // The GUI we will see in the inspector
 }

When I go back to unity, I get the following two errors:

I don't understand what I am doing wrong. Thanks for the help!