I’ve run into this issue multiple times for various reasons when using Magento’s dataflow importer. In my research of the problem I came across https://www.mag-manager.com/useful-articles/case-studies/error-skip-import-row-required-field-not-defined/ that contained many helpful suggestions, but none of those solved the issue. Just a few products were causing this issue during the import on my dev environment. I found out the product did not exist in my older copy of the database, which caused this error. So much for having a meaningful error message :).