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.
The most common way to add a page to the tree is to create it specifically as a child of an existing page. This ensures it is nested correctly from the very first keystroke.
For moving pages across different spaces or complex hierarchies: Open the page you want to move. Click the in the top right. Select Move . Search for the New Parent Page or select a New Space . Click Move to confirm. Organizing the Tree with Reorder Pages