Please follow the below thread, I have explained the issue in detail:
https://stackoverflow.com/questions/69243728/scriptablesingleton-unity-return-0-values
ScriptableSingletons are explicitly for the Editor and I’m certain their serialization is quite different. Your use-case also feels weird for a singleton, surely you’d just link the PlayerAbilities SO to your GO that needs the data?
It says what it actually is,
he cannot find the Singleton Class, you need to import it 
and why a singleton SO?
havent seen this use case, im curious 
I was trying to use ScriptableObject as singleton because I was refering it in different places dynamically without being dependent to drag and drop the Scriptable Object into the Player Inspector hierarachy.
So when I was referring it dynamically, I were to create instances every time when I try to access its members.
Therefore I went ahead to make it as singleton and initialize it at the loading the scene, so now I can access its members anywhere anytime.
What your describing is a dependency issue. There are a lot of different ways to handle that. Singeltons tend not to be one of the good ways.
Google dependency injection in unity and you might find some better solutions