Cause 1: There is an inventory item with no Code or a description that does not match the existing code in QuickBooks and QB is rejecting that.
Remedy 1: Check your inventory and make sure all items have a Code and Description. Check QuickBooks and ensure that it matches and also that the account hasn't changed.
Cause 2: The inventory item is marked as a Service in QuickBooks and as a Product in Kennel Connection. This can happen if it was previously a service in KC and then changed to a product after the initial sync to QB.
Remedy 2: Compare the items in KC and QB and set it properly in KC so it matches QB.