Problem:  Getting the above Error Code when I try to modify a listing

Solution:  "Ebay will NOT allow you to remove the best offer option on a listing that has any sales history to it. You will need to end and relist the item if you want it removed."  So even if there is not a pending offer you will still get this error if the listing has an sales history on it at all, even if it is a GTC FixedPrice listing.