Usually, in the Customize Preview, you are able to see only the pages that are linked, but if you have a page that is not linked anywhere on your website, it will be very hard for you to see it in the Customize.
If you need to see a specific page in the Customize preview in any case, you can install All Pages IIn Customize.
After activation, if you go to Appearance => Customize, you will see a dropdown list for each post type that is publicly queriable.
