PLEASE NOTE: eBay errors are common. If your item is showing "Failed to List" after attempting to list, you can learn more by clicking on the item receiving error to see what eBay is asking from your listing under 'eBay Status'.
This error occurs when your item is being categorized to the wrong region. For example, it could have been put in a US item category, instead of the UK equivalent. Fix this by manually searching an appropriate category when going to list in the pop-up listing tool.
If you're still receiving the error, search the item you are selling directly on eBay and look at what categories competitors have placed their items. Link them to the eBay category directory as well.