If you cannot drag a page, check your permissions. You must have "Add" permission in the space to create child pages. If the space is locked down by administrators, you may not be able to alter the tree structure.
To add a page to a page tree in Confluence, follow these steps: confluence add page to page tree
This guide covers the various methods to add a page to a page tree, from creation to reorganization. If you cannot drag a page, check your permissions