You never created an instance of your C1 class inside your Main method. That's what it means. You're trying to use a normal instance class like a static class. You can't do that.
You have a constructor on the C1 class that requires a bunch of parameters so you can create an instance of the C1 class.
You have to collect all of your information first, the create an instance of the class, then you can use the properties and methods of the class.
static void Main(string args)
double variable name;
Console.Write("Enter Loan Amount: ");
C1.LoanA = Convert.ToDouble(Console.ReadLine());
Console.Write("Enter Years: ");
C1.Years = Convert.ToDouble(Console.ReadLine());
Console.Write("Enter Interest Rate: ");
C1.InterestR = Convert.ToDouble(Console.ReadLine());
C1 instance = new C1(parameters go here);