In order to fix this you must change the item back to what it was when the listing ended.
Most of the time this will be stock that the customer changed.
For example:
--if the stock now says 25 and you are getting this error, you must change the stock back to 0 and click Update in the bottom action bar of the item editor.
--At this point you can add back in the 25 and then use the Relist/Publish Item button (or choose End Listings by Channel from the '- Select Action -' dropdown, then relist) to retry sending the item to the channel. The button previously labeled 'RETRY CHANNEL RELIST' has been replaced by these controls in the current item editor.
--If you get another error, fix it as normal and then try the relist action again; that will list the item as intended.