Switch to App studio and click on the preview button
Click on the Design button on the right side
Click on the Pages Icon and click on New button
Provide a sample page name and click on submit.
Exit preview and click on Channel and select any of the channel interface
Click on Add Item and select the page created and click Save.
Now go to Dev studio and open the portal we have configured the navigation page.
we should able to see the newly added page in navigation.
Reference:
https://academy.pega.com/topic/creating-page-application/v2/in/1341