Custom Editor script to show non-native class in inspector

So I have a class that is not extending anything (apart from System.Object of course) and therefore variables of that type don't show up in the inspector. I tried writing a CustomPropertyDrawer, but it still doesn't show the variable. This is my code:

 using UnityEditor;
 using UnityEngine;
 
 [CustomPropertyDrawer(typeof(AScriptableObjectClassThatShouldDisplayTheVariableInItsInspector))]
 public class ShowVariable : PropertyDrawer
 {
 
     public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
     {
         base.OnGUI(position, property, label);
 
         EditorGUI.BeginProperty(position, label, property);
 
         Rect rect = new Rect(position.x, position.y, 50, position.height);
 
         EditorGUI.PropertyField(rect, property.FindPropertyRelative("nameOfVariable"));
 
         EditorGUI.EndProperty();
     }
 
 }

This is the class that contains a variable of the class that should be displayed:

 [System.Serializable]
 public class Syntax : ScriptableObject
 {
   public GameLevelFunction nameOfVariable;
 }

And this is basically how my class that should be displayed in the inspector looks like:

 [System.Serializable]
 public abstract class GameLevelFunction
 {
   //literally two methods
 }