You must first collect traffic on the page in order to declare a page custom variable for it.
If you can not have traffic on that page, you can generate a hit like this:
This hit should be adapted to include the correct log, site number, level 2 site and page name (including chapters). Please refer to our online documentation to know more about manually crafting hits: https://developers.atinternet-solutions.com/general-en/craft-your-hit/
Once you send the hit, the page label will appear in the interface and you will be able to declare new custom page variables for it.