This could be caused by a few things; for some reason, eBay thinks you have an item too similar to this already for sale.

First, please check your eBay and make sure this item isn't listed there. If it was ever relisted directly on eBay, or somehow disabled in SureDone, it may have fallen out of sync.
   - If this is the case, end the list on eBay directly and relist it through SureDone, this will make sure it stays synced with your current inventory levels in SureDone.

Another common reason is that you have an item too similar in title and/or description. 

   - Try to just change the title and/or description around a little bit and then click retry channel relist.