It's not possible to associate a page to a variant.

Reproduction

1. Have a book, with version and variants set
2. Create a page
3. Select a variant in the dropdown menu
4. Open the "Versions" menu on the top right, and see that there is no way to link the page to the current selected variant

Workaround

1. Edit the page objects
2. Add a VariantsListClass object
3. Select the variants to which the page will be associated