UnityEditor related classes can not be referenced. Why?

I have two classes. Both of them are placed in the editor/interface_field folder.

using System;
using UnityEngine;

namespace editor.interface_field
{
    public class RequiredInterfaceAttribute : PropertyAttribute
    {
        public readonly Type type;

        public RequiredInterfaceAttribute(Type type)
        {
            this.type = type;
        }
    }
}

    using UnityEditor;
    using UnityEngine;
    
    namespace editor.interface_field
    {
        [CustomPropertyDrawer(typeof(RequiredInterfaceAttribute))]
        public class RequireInterfaceDrawer : PropertyDrawer
        {
            public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
            {
                if(property.propertyType == SerializedPropertyType.ObjectReference)
                {
                    var requiredAttribute = this.attribute as RequiredInterfaceAttribute;
    
                    EditorGUI.BeginProperty(position, label, property);
    
                    property.objectReferenceValue =
                        EditorGUI.ObjectField(position, label, property.objectReferenceValue, requiredAttribute.type, true);
    
                    EditorGUI.EndProperty();
                }
                else
                {
                    var previousColor = GUI.color;
                    GUI.color = Color.red;
    
                    EditorGUI.LabelField(position, label, new GUIContent("Property is not a reference type"));
    
                    GUI.color = previousColor;
                }
            }
        }
    }

I have another class.

using editor.interface_field;
using UnityEngine;

namespace interface_user.implementations
{
    public interface IInterface {}
    public class InterfaceUser : MonoBehaviour
    {
        [SerializeField]
        [RequiredInterface(typeof(IInterface))]
        UnityEngine.Object interfaceImplementation;
    }
}

The [RequiredInterface(typeof(IInterface))] line throws the following error:

The type or namespace name 'RequiredInterfaceAttribute' could not be found (are you missing a using directive or an assembly reference?) [Assembly-CSharp]csharp(CS0246)

I can not understand why does not Unity see the RequiredInterfaceAttribute class and how could I make it see it.