Unless there's less than 12 hours left to run on the auction, in which case bids can't be cancelled, I suspect it's a temporary glitch, so try again later. If still no luck, what browser are you using...? If IE, note that it's no longer compatible with Ebay or any other website (Microsoft have replaced it with Edge). I use Firefox with no problems. Or if you're using the app, ditch it and use the desktop version instead.
If you continue to have problems, everything you need to know about selling on Ebay, including how to cancel bids, is in the Seller Centre (link at foot of any page) and in the Help pages (links at top and foot of any page).
@29rgm
