Using CustomEditor caused error which was CS0246.

I have two classes of my project. First one is some data to show up.

 using System;
 using UnityEngine;
 
 public class InspectorTest : MonoBehaviour
 {
     public int a;
     public float b;
     public string c;
     public DataBase d;
 }
 [Serializable]
 public class DataBase
 {
     public int a;
     public float b;
 }

Second one is used to exhibit those data onto inspector.

 using UnityEditor;
 
 [CustomEditor(typeof(InspectorTest))]
 public class InspectorEditor : Editor
 {
     public SerializedObject inspectorTest;
     public SerializedProperty aInteger;
     public SerializedProperty bFloat;
     public SerializedProperty cString;
     public SerializedProperty dDataBase;
 
     private void OnEnable()
     {
         inspectorTest = new SerializedObject(target);
         aInteger = inspectorTest.FindProperty("a");
         bFloat = inspectorTest.FindProperty("b");
         cString = inspectorTest.FindProperty("c");
         dDataBase = inspectorTest.FindProperty("d");
     }
 
     public override void OnInspectorGUI()
     {
         base.OnInspectorGUI();
         EditorGUILayout.PropertyField(aInteger);
         EditorGUILayout.PropertyField(cString);
         EditorGUILayout.PropertyField(bFloat);
         EditorGUILayout.PropertyField(dDataBase);
         inspectorTest.Update();
     }
 }
 

When I typed InspectorTest into "typeof", the compiler done nothing and without any tips. Then caused the error was CS0246. I tried to save my code in VS. The error was remaining here. So I turned to unity and figured out there was no error in console panel but same properties in inspector. First one is located in "Assets/Scripts/Attribute", second one is located in "Assets/Scripts/Attribute/Editor". My Unity version is 2020.3.24.f1c1. How could I fix it?