How come I cannot set my prefab game component into a script public variable?

I'm following the 2d Roguelike tutorial. In step 13 adding the sounds, I created a sound manager script in the scene. I then made it a prefab with the script and the 2 audio sources in the as a game component. I then added the SoundManager to my loader script, but I cannot drop in the prefab sound manager game component into the loader. The weird part is when I drag the prefab onto the public script variable in Loader in the Unity UI it highlights as a good link but doesn't update when the mouse is released.

My scripts are the following.

  public class Loader : MonoBehaviour
     {
         public GameManager GameManager;
 
         public SoundManager SoundManager;
 
         static Loader()
         {
             log4net.Config.BasicConfigurator.Configure(new ConsoleAppender(new SimpleLayout()));
         }
 
         private void Awake()
         {
             if (GameManager.Instance == null && this.GameManager != null)
             {
                 Instantiate(this.GameManager);
             }
             if (SoundManager.Instance == null && this.SoundManager != null)
             {
                 Instantiate(this.SoundManager);
             }
         }

 public class SoundManager :  MonoBehaviour
     {
         public static SoundManager Instance;
 
         public AudioSource EfxSource;
 
         public AudioSource MusicSource;
 
         public float LowPitchRange = .95f;
 
         public float HighPitchRange = 1.05f;
 
         public void PlaySingle(AudioClip clip)
         {
             this.EfxSource.clip = clip;
             this.EfxSource.Play();
         }
 
         public void RandomizeSfc(params AudioClip[] clips)
         {
             var randomIndex = Random.Range(0, clips.Length);
             var randomPitch = Random.Range(this.LowPitchRange, this.HighPitchRange);
             this.EfxSource.pitch = randomPitch;
             this.EfxSource.clip = clips[randomIndex];
             this.EfxSource.Pause();
         }
 
         private void Awake()
         {
             if (Instance == null)
             {
                 Instance = this;
             }
             else if (Instance != this)
             {
                 Destroy(this.gameObject);
             }
             DontDestroyOnLoad(this.gameObject);
         }
     }