Classes are the computer representation of things in the real world, and - just like in the real world - you need instances of a class in order to use them. That probably doesn't make a lot of sense right at the moment, or have any relevance to your error message - but bear with me ...
Instances are all around us, so let's ignore computers for a moment and talk about cars.
This car; that car; your car; my car. We all know what they mean: they identify a specific vehicle out of the "phase space" of "all potential cars" that I could be discussing. When I say "I drove my car to the shops" you envisage me at the wheel of a large grey Mercedes (or you would if you knew me) rather than the compact blue Ford you normally drive. "My car" here serves to indicate specifically which car I am talking about without confusion.
"My car" references an instance of a car, while "Your car" references a different instance. And instances can be "shared": if my wife and I have just one car which we both drive, we can refer to it as "Our car", "My car", and "Her car" without confusion. The references identify the same instance.
And we all know that cars are different objects: If you put your mobile in the glove box of "your car", you wouldn't expect to open the glove box of "my car" and find it. But...if my wife puts her mobile in the glove box of "her car" then I can find it for her by looking in the glove box of "my car". Same instance, two references.
And we know that you need an instance of a car to do anything: you can't ask the question "What colour is a car?" because not all cars are the same colour. We can ask "How many wheels has a car?" because all cars have four wheels - if they had less, they would be a trike or bike!
When you take this back to computers, you find you have a similar situation.
Suppose we have a simple little class:
public class Car
{
#region Properties
public string Manufacturer { get; private set; }
public string Model { get; private set; }
public Color Colour { get; private set; }
public static int NumberOfWheels { get { return 4; } }
#endregion
#region Constructors
public Car(string manufacturer, string model, Color colour)
{
Manufacturer = manufacturer;
Model = model;
Colour = colour;
}
#endregion
#region Overrides
public override string ToString()
{
return $"{Manufacturer} {Model} ({Colour})";
}
#endregion
}
Now we can create something that describes my car:
Car myCar = new Car("Mercedes", "B180", Color.MountainGrey);
This line of code does several things:
It creates a variable called "myCar" which can hold a Car (or a class derived from Car).
It creates a new instance of a Car, gives it the information that identifies it as my car.
The instance is assigned to the variable.
We can create a different car:
Car gillsCar = new Car("Volkswagen", "Golf", Color.Orange);
And it's pretty obvious that that is a separate vehicle!
So when we want to know what colour a vehicle is, we "ask" the appropriate vehicle:
Console.WriteLine($"I drive a {myCar.Colour} car.");
Console.WriteLine($"Gill drives a {gillsCar.Colour} car.");
And we will get the right data:
I drive a Mountain Grey car.
Gill drives a Orange car.
But we can ask about wheels by querying the Car class directly:
Console.WriteLine($"A car has {Car.NumberOfWheels}.");
And get
A car has 4 wheels.
If I try to ask "What colour is a Car?" then the code looks like this:
Console.WriteLine($"A car is coloured {Car.Colour}.");
And I get an error:
An object reference is required for the non-static field, method, or property 'Car.Colour'
Because not all cars are the same colour!
And now we go back to your code where you have the same error: you need to specify exactly which deck of cards you are trying to shuffle, because not all decks are the same: two blackjack tables in the same casino will each be drawing from their own deck which do not interact!
So your code needs to declare a variable to hold a deck of cards, and use that to shuffle and deal from.
I can't write that code for you - I can't see enough of your project to do that - but hopefully now you understand a little better what you are trying to acheive, and how you need to achieve it. Make sense?