The answer is to send products to the channel as single, after creating an alias of it.
We all had this problem; our inventory is set up as parent-child products, or finally you made the effort to consolidate all of your products as Parent-child, but upon sending listing to Amazon we get an error message saying that product category does not support listings with variation.
There are a couple solutions to this problem:
You can create an alias of all children with a new SKU and send these new products to the channel. An alias of a child product will be 'single'. You can send this alias and get around Amazon's requirements. Variation options such as size or color can be sent in a custom field.
You can find a similar category that allows parent-child relationship. This takes a little more time and research to find a similar category in Amazon.
Creating an Alias
StoreAutomator features Alias creation to come over this problem. An Alias of a product is basically the same product as its master, with an option to change a number of fields. They share the same inventory; if an alias is sold the number comes down from the master product. An alias of a 'parent' or 'child' product is always single. You can also assign and limit number of inventory for an alias.
Differences between Alias an Alt SKU
Alias and Alt SKU are basically the same method, except Alt SKU allows to change the SKU only, while with Alias you can change all product fields. They both share inventory with their master, they represent the same product.
Finding a similar category is also an option. In this case you need to sit down, kick back, login to you Amazon Seller Central account and find a suitable category from create anew product. You can also view the latest Amazon spec files and look up from there.