Since Confluence version 6.7.0 the Metadataset can no longer be added when creating a new page.
But it works for all Confluence versions lower than 6.7.0.
When creating the new Confluence page, you need the Collaborative Editing Mode.
So we check the Collaborative Editing status:
The problem is solved for the modes "On" and "Limited".
You can also solve the problem as follows: