SKU does not exist in Non-ManageBySKU item specified by ItemID

The error means there is a mismatch between the SKU of your Magento Product and the SKU of the linked Item on the channel. 

One of the most common reasons why you could be getting this error is that you are trying to revise a variation that has already been removed from the Channel Item. For example, you listed a product with a variation ‘Size S’ with quantity 7. Later, you revised it with quantity 0. And when you update a variation with 0, it gets removed from eBay. Now you are getting the error because you are attempting to revise the same variation ‘Size S’ with a certain quantity. 

To get rid of the error and update the product, please make sure your Magento product and the Item on the Channel have the same SKUs.