15th Anniversary - Korrak's Revenge - "Join as a group failed"

[Punyelf-draenor (Punyelf)]

Blue post in GD suggests they are aware and working on the issue:-

Korrak's revenge, why can't we queue for it?

Hey everyone! We understand you’re all excited to experience the Anniversary content. We’re currently looking into these queuing issues and we hope to have them resolved as soon as possible .