Making a Serializable class that doesn't throw "The class named 'Legacy' is not derived from MonoBehaviour or ScriptableObject!"

I have the functionality I want. My issue is that I'm getting the warning "The class named 'Legacy' is not derived from MonoBehaviour or ScriptableObject!" I want the warning to go away. It is an accurate warning, because my class, Legacy, does not extend Monobehaviour. I did this so that I may tag it as [Serializable]. I'm trying to learn about game saves and serialization from this Unity live training video.

The difference I can see between my code the trainer is that my Legacy class is in a separate file and I instantiate an array of type Legacy from my GameManager script, which does extend Monobehaviour. In the live training, the class that is serialized is a private class and he does not get this error.

Here is my Legacy class:

 [System.Serializable]
 public class Legacy {
     
     public int id;
     public string name;
     public int fame;
 
     public Legacy( int id, string name, int fame) {
         this.id = id;
         this.name = name;
         this.fame = fame;
     }
 
     public void AddToLegacyManager() {
         LegacyManager.legMan.legacies[id] = this;
     }
 }
 

And here is where I hardcode a few elements (within my a class derived from Monobehaviour):

 void CreateLegacies() {
 
         //Create legacy for each player
         Legacy leg1 = new Legacy( 0, "Achilles", 4);
         Legacy leg2 = new Legacy( 1, "Hercules", 2);
 
         leg1.AddToLegacyManager();
         leg2.AddToLegacyManager();
     }

And here is where I serialize in LegacyManager:

     public void Save() {
         BinaryFormatter bf = new BinaryFormatter();
 
         FileStream file = File.Create( Application.persistentDataPath + "/playerInfo.dat");
         
         Legacy[] legs = new Legacy[2];
         legs[0] = legacies[0];
         legs[1] = legacies[1];
 
         bf.Serialize( file, legs);
         file.Close();
         
     }
 
     public void Load() {
         BinaryFormatter bf = new BinaryFormatter();

         if( File.Exists( path + "/playerInfo.dat")) {
             FileStream file = File.Open( Application.persistentDataPath + "/playerInfo.dat", FileMode.Open);

             Legacy[] legs = (Legacy[])bf.Deserialize( file);
             file.Close();
 
             legacies[0] = legs[0];
             legacies[1] = legs[1];
         }
     }
 }