You can't "solve" an in put string format problem: it says what it means: the user typed a value that cannot be interpreted as a valid
double
You can prevent you app from crashing though:
double R;
while (true)
{
Console.Write("Please enter the radius of the circle :");
string input = Console.ReadLine();
if (double.TryParse(input, out R))
{
break;
}
Console.WriteLine($"\"{input}\" is not a valid number.");
}
double A = Math.PI * Math.Pow(R, R);
Console.WriteLine("A = ", A.ToString("F4",CultureInfo.InvariantCulture));