EBay can be very buggy and glitchy. The best you can do is probably wait a few hours and try again.
Or try another route, for example find a similar item on eBay to the one you want to sell and use the drop-down box at top left to choose "Sell Similar". That should take you to the "Sell Your Item" (SYI) form. But watch out for shipping, location, or other information that was specific to the original item in the listing details and change it to reflect your own details, in other words, check through everything very carefully before you list.
