Hi,
note the page with the [shortcode_payments_complete] shortcode should not be accessed directly, as it will work only if the URL includes a special hash which identifies a payment and allows to complete it.
So the use case for this shortcode is the following: a user post a paid ads but forgets to make a payment or closes the browser window before making a payment, the administrator can then go to the wp-admin / Classifieds / Payments History panel edit his payment, copy the complete payment URL and send it to the user (the URL will be something like https://example.com/page/?advert-hash=xyzxyzxyzxyz…) and the user can complete the payment from there.
In a normal use case when user posts an Ad (from a page with Classifieds Publish block or [adverts_add] shortcode) you do not need that page at all, user will be able to submit an ad and complete payment without it.