There should be a "Variant Inventory Policy" column in the CSV. This is what it does.
How to handle orders when inventory level for this product or variant has reached zero. Valid values are "deny", or "continue". "deny" will stop selling when inventory reaches 0, "continue" will allow sales to continue into negative inventory levels.
It is a required field if you are tracking inventory.
There is a bunch more information about each of the different columns in this doc. Definitely check it out : http://docs.shopify.com/manual/your-store/products/product-csv
Andrea M | Shopify Guru
I am having the same issue, however I have properly filled out the column for the inventory policy. I still receive the following error prompt:
Here is a link to my Google Sheets file: