Hi,
“post is not a product” error occurs due to a post already exist in the importing site with the same ID.
Wordpress considers everything as posts. Eg: product is a post, the image is another post Like that.
So if you upload a product that has an image with product ID 4000, then WordPress assigns 4000 to the image. Conflicts occur if you give consecutive IDs for products.
For product import, it is always preferable to avoid using consecutive IDs.