Full Error Message:
Your eBay account already contains a product with this SKU. All SKUs must be unique within your active and scheduled listings. Please make sure you are using unique SKUs.
To allow ChannelUnity to match to existing listings on eBay by SKU, in the menu click on Settings -> Channel Settings -> Link my Listings. Click on “Complete” on the left side. This will match products, and display any products already on eBay as Listed in ChannelUnity.
Error: When specifying SKU for InventoryTrackingMethod, the SKU must be unique within your active and scheduled listings
ChannelUnity it trying to create a new eBay listing but the SKU already exist and in use in the eBay account. It could be the parent and/or the variation SKU contained in the Custom Label field.
To allow ChannelUnity to match/link-up to the existing eBay listings on eBay by the SKU, then select “Settings -> Channel Settings -> Link To Existing Listings” and follow the linking process.
Delete and purge the listing in ChannelUnity which will send a delete request to eBay to remove the listing, wait 20 minutes, and then upload the product to create a new listing on eBay (note: the old listing will be deleted from eBay with all it’s history).