Because print_address() is how one would call a subroutine called print_address. Apparently there is no such subroutine, thus the error. You need to create the subroutine or remove that line.
print_address is not a built in function so you would have to supply it yourself. If you provided a link to the tutorial you are using it would be easier to help you. Try just changing it to print instead.
P.S. if you want to include code in your post wrap it in <code></code> tags.