Had the same problem myself recently. When I clicked on the 'print label' link the pop up only contained my own address - this was on my works laptop, chrome browser. Came home sent them a message on twitter, got a message back to try again and this time it worked - was on my home pc using edge browser. So not sure if it was a browser issue or the person at the other end of tweet 'fixed' something.
You could also have a mini (but polite) rant I did and they issued me a prepaid returns label.