Guest purchases are not connected to your eBay account.
For help with a guest purchase, sign out of your current eBay account and use the link in the email that eBay sent you regarding the order, or else see here:
https://www.ebay.com/help/buying/returns-refunds/returning-item/returning-items-bought-guest?id=4065
If you still have problems, you can sign in to your account and contact eBay and explain that you are having problems printing the guest return label.
Use the Help & Contact link at the bottom of the page to request a call back from eBay. Use the "Contact us" link and follow the prompts for your issue and you should eventually wind up on a page with a chat option. Choose that, and the last option that appears in the chat window -- use the "See more" option -- should be "Contact an Agent" to request a call-back if you are in North America.
https://community.ebay.com/t5/Selling/How-do-I-contact-Customer-Support/m-p/32016431#M1783851
If using the "Contact us" link takes you directly to the chat assistant window, you may have to enter some context dependent cues to get to the call back link.
Or you can contact eBay via social media:
https://twitter.com/askebay
https://www.facebook.com/eBay/
