The error means what it says: a variable called
is already declared in the method, so you cannot declare another new one - or you would end up with two variables with the same name, and while the compiler wouldn't get confused between them a human reader would!
It's generally a good idea to avoid single character variable names anyway and use more descriptive ones:
for (int fileLineNumber = 0; fileLineNumber < writeFileContents.Length; fileLineNumber++)
writeFileContents[fileLineNumber] = Console.ReadLine();
Console.Write("Line " + fileLineNumber + ": ");
It helps your code become more self-documenting, as well as more maintainable - particularly when you have multiple nested loops!
Keep single character variables for "quick values" which will be used once and can't be easily confused - such as a loop counter that doesn't actually care what loop iteration it is on:
for (int i = 0; i < 16; i++)
Console.Write(inputValue & 1);
inputValue >>= 1;