example1.cs

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main(string[] args)
    {

        var list = new List<Skill<DifficultyPoint>>();

        var reading = new Reading();
        var test1 = new Skill<ReadingPoint>();
        var test2 = new Skill<DifficultyPoint>();
        
        list.Add(test2);
        list.Add(test1); // Argument 1: cannot convert from 'Skill<ReadingPoint>' to 'Skill<DifficultyPoint>'
        list.Add(reading); // Argument 1: cannot convert from 'Reading' to 'Skill<DifficultyPoint>'

        Console.WriteLine(list);
    }
}

class DifficultyPoint {}
class ReadingPoint : DifficultyPoint {}

class Skill<TDiffPoint> where TDiffPoint : DifficultyPoint {}

class Reading : Skill<ReadingPoint> {}

example2.cs

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main(string[] args)
    {

        var list = new List<Skill<ReadingPoint>>();

        var reading = new Reading();
        var test1 = new Skill<ReadingPoint>();
        var test2 = new Skill<DifficultyPoint>();
        
        list.Add(test2); // Argument 1: cannot convert from 'Skill<DifficultyPoint>' to 'Skill<ReadingPoint>'
        list.Add(test1);
        list.Add(reading);

        Console.WriteLine(list);
    }
}

class DifficultyPoint {}
class ReadingPoint : DifficultyPoint {}

class Skill<TDiffPoint> where TDiffPoint : DifficultyPoint {}

class Reading : Skill<ReadingPoint> {}