I tried troubleshooting what you had and after commenting out a few sections, the square_list just included the value "4" for some reason. No idea why x=4. I think where you're running into trouble is the print function. The print function will cause the for loop to just output the squared start_list instead of placing the squared values in the square_list.
Try replacing the print function in the
for x in start_list:
portion with the append feature. Think of it as the for loop outputting directly into the square_list, if that helps. After this (should be just 2 lines of code), you can sort and print. Hope this helps!