I had this error, after an Magento 1 migration to Magento 2. Then tried to import using a really simple CSV with 4 new attributes.
My issue was there was an attribute called `product_type` that had been migrated across from Magento 1. In Magento 2 , it seems `product_type` is a reserved system attribute, and should be simple, configurable, bundle, etc.
To fix it, i deleted the product_type attribute that shows up in Store > Product > Attributes and then it worked
