You may receive this error on some products as eBay does not allow the name of the variation.
For some categories, eBay does not allow certain words to be used as the name for variants (most often because they have been reserved as Item Specific filter values for that category).
For example, the word “Size” is not allowed as a variant name for the eBay category “Baby » Nursery Bedding » Sleeping Bags & Sleepsacks”.
This can be confirmed by attempting to list an item on eBay manually using the list an item process, selecting the category in question and attempting to add your own variation attribute named “Size”. See screenshot below.
Resolution: An acceptable name for the variation in this category as a replacement for size would be: "To Fit". You will need to change the name of the variations in your ecommerce platform.