The page_impressions_organic_v2 and the page_impressions_organic_unique_v2 fields recently started to fail.
According to the documentation the fields still exist and should work: https://developers.facebook.com/docs/graph-api/reference/page/insights/
Why did these fields start to fail?
This metric is deprecated. It is listed in the changelog: https://developers.facebook.com/docs/pages-api/changelog/