That means that you've tried to access a variable named
string_to_check but you have not defined such a variable.
Nowhere in your code have you assigned a value to that variable:
string_to_check = some value
Look at your code, you have used a different variable name,
user_input, to hold the value that we are checking, which is perfectly fine. Because you have defined that, it is the one that you should be using.
Do you see the sense in that now?
One more thing for you to look at. There is no reason for you to try to assign your print statement to a variable.
string = print ("thrings pleathe: ")
print "thrings pleathe: " // isn't this nicer?