I created the following class:

public class Check<TInput> : CheckBase<TInput, (DateTimeOffset Stamp, Decimal? Value), Reply> where TInput : ITicker {

  public override IEnumerable<Reply> Compute(IEnumerable<(DateTimeOffset Stamp, Decimal? Value)> values) {

     // Just for testing
     return new List<Reply>();
        
  } 

  public class Reply(DateTimeOffset stamp, Decimal? value) : ITicker {

    public DateTimeOffset Stamp { get; set; } = stamp;
    public Decimal? Value{ get; set; } = value;
    
  } 

}

public abstract class CheckBase<TInput, TOrder, TReply> : IIndicator<TInput, TOrder, TReply> where TInput : ITicker where TReply : ITicker {

  public abstract IEnumerable<TReply> Compute(IEnumerable<TOrder> orders);

}

public interface ITicker {
                 
  DateTimeOffset Stamp { get; set; }

}

But I get the error:

The type or namespace name 'Reply' could not be found (are you missing a using directive or an assembly reference?)

In the Check class definition:

... Reply> where TInput : ITicker 

Why?