What error or message do you see when you try to list?
If it's a "connect" error, that's an ongoing glitch - you can post over on the Tech Issues board for more help, or see if any solutions there.
Possibly signing out, clear cache/cookies, restart and sign back in may work (so I've read)
https://community.ebay.com/t5/Share-eBay-Technical-Issues/bd-p/technicalissues-db
