Due to some problem in page, Page Layout button in ribbon is showing disabled.
Use following as QUICK solution if you are creating a new page:
- Delete newly created page
- Go to site settings > Page Layouts and Templates
- Locate "New page defualt settings" and select the page layout that you want to set as default page layout.
- Save setting
- Create new page
There can be many reasons of Page Laout showing disabled in ribbon. You can reference following links to go deeper and resolve it properly.