Simple 5 operator calculator in C#
Remove the useless functions
There's already a sum function. It's called +
Instead of
Console.WriteLine(Sum(n1, n2));

and
static Decimal Sum(Decimal n1, Decimal n2)
{
    Decimal sum;
    sum = n1 + n2;
    return sum;
}

just write
Console.WriteLine(n1 + n2);

... and the exact same thing applies to all the other calculation functions you wrote.
The same thing in every branch of an if
No matter what op is, you do Console.ReadLine();. So delete it from each if/else if/else and just add it after the end of the if.