How do I continually ask to enter a new input or stop the program from running?

I have been trying every single thing I could think of but no matter what I get stuck in an infinite loop. I want the last variable of userInput to either take the string of 'yes' so it will repeat the function or take the string of 'no' to stop the program from running. This is as close as I can get to the finished product. I need the box asking yes or no to pop up every time after the function runs once. Attached is an image of my code.