Can I still use POP if my page has not been published yet?

Yes. You can use POP before the target page has been created or published.

When setting up the report, select the option indicating that the page has not been built yet. In POP Custom this appears in Step 1, while in POP Pro it appears beneath the Target Page Path field.

POP will then generate recommendations that can be used while creating the new page.

If you work with a writing team, you can download the recommendations for your writers. Unlimited and Teams plans can also provide access through the POP Content Editor or a report Secret Key for use in Google Docs or a supported website editor.

If the domain itself is not live yet, the source article says you can still create the project using the future domain name or a temporary dummy domain.