Please use code tags in the future. They are essential here, and most of the time (and still important the rest of the time).
Your program says, "while x is less than 20, assign to the variable xString the user input". Since what you're doing inside the loop doesn't change the exit condition, it doesn't ever exit. All the code after your single indented line follows the loop, rather than being inside of it. If you want the code to be inside of the loop, indent it to the same level as the line that is in the loop.
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