This page is not using a valid page layout. To correct the problem, edit page settings and select a valid page layout.

Problem:
We had a requirement to copy few web part pages from Staging environment to production. Achieved it with "Content Deployment Wizard" tool. However, copied pages gave this error message:
"This page is not using a valid page layout. To correct the problem, edit page settings and select a valid page layout."

Solution:
Root cause of the problem is: Page layout URLs are hard-coded in the publishing/web part pages. When you copy/move them from one site to another (or during export-import).

Solution is: You'll have to update it. Here is the easiest way:

1. Open you site in SharePoint designer, Navigate to the library (usually: Pages), Right click on your copied page, choose "Detach from page layout"
   
2. Now, again right click on the page and choose "Reattach to Page Layout". Ignore any error it throws.

This should fix your problem. If you have a bunch of files to update page layout, You can use PowerShell. Here is my PowerShell script to update page layout property of pages: How to Change Page Layout in SharePoint 2013 using PowerShell