Did you encounter this warning message, while you are applying to Join WordAds program with your “Premium” website/blogs account?
Did you also notice the “Not secure” text in your website https:// location as mentioned below ?
This means, your site is still running on http which should be shifted on https after upgrading/purchasing your site with premium/e-commerce plan automatically.
Now, you need to check all the widgets and HTML code where, you are using any URL running with http. if this is the case, you must need to change into https and then browse the website URL, you should see valid certificate lock icon in URL.
in my case, i was using cluster MAP which was using http:// for its URL and I replaced into https:// to resolve this problem.
Now, you can go back and apply for wordads program or simply you can chat with wordpress chat support to do it for you.