C# return isn't letting user know of invalid entry

I have public string method with an if/else statement. When I don't put a return keyword on the else, I get the error that not all code paths return a value but when I add the return to the else statement, I get an Use of unassigned local variable 'result'. When I do return "Please enter a valid number";, nothing displays on the console when I type in any letter. I need to let the user know that they entered something in that couldn't be converted into an integer.

public string AddNumbers(string userInputString)
    {
        int result;
        int num2 = RandomNumberInt();
        bool isTrue = int.TryParse(userInputString, out int num1);

        if (isTrue == true)
        {
            result = num1 + num2;
            Console.WriteLine("Adding your number and a random number, please wait...");
            Thread.Sleep(1000);
            Console.WriteLine("{0} + {1} = {2}", num1, num2, result);
            isTrue = true;
            return result.ToString();
        }
        else
        {
            return "Please enter a valid number";
        }
    }