Error:
'EtsyError: [{"path":"/marketplace","type":"invalid_marketplace","message":"Oh dear, you cannot sell this item on Etsy."}]'
This error occurs when you set the who_made column to someone_else without providing a production partner id in the production_partner_# columns.
Solution:
Enter a production partner id into the column.
You can find these on the Shop page.
If you do not have any production partners and need to make one please visit the How to Create an Etsy Production Partner article.