Not all my (wordpress) pages validate. Some pages seem to be missing: twitter:description. I use twitter card metadata plugin.
Not sure if this helps, but you can read here: https://dev.twitter.com/docs/cards/cms-integration
It might help with your specific plugin, or you can configure jetpack (if you’re self hosted)
Thanks a lot for quick reply. I deactivated the Twittercard feature from the Yoast SEO plugin. Twitter card metadata is still activated. Tried to validate, two more pages seem to be working. Maybe it takes some time, although site is approved.