Nooo, you must never cancel a return case as another for the same item cannot be opened.
All I can suggest is to contact Customer Services explain what has happened and that you need a new and correct label for the return.
Hopefully, they'll reopen the case and send the correct label. No guarantee it may depend on the agent you get, but your only way forward here.
I strongly recommend contacting CS 8 a.m. on a weekday when the lines open for ' live agents' as then, there's more chance of Dublin answering.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5190
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
@terrano27