I am not sure how you have it setup exactly?
Have you created a new page from wp-admin / Pages panel withy the [adverts_payment_checkout] shortcode in the content?
If you did then how is this page accessed? It should only be accessed with some adverts-hash param in the URL, otherwise, it will not be able to figure out which payment you want to pay.
Greg apologizes for my ignorance: Where should I put the code [adverts payment checkout] so that users can process the payment by bank transfer?
By default, you do not need to use this shortcode at all, WPAdverts can work without it, especially if you are working with Bank Transfer payment method which does not have any way to complete the payment online.
If you want to use it then go to wp-admin / Pages panel and create a page which will be used for completing payments.
This URL should be accessed only with a correct adverts-hash param in the URL, thee urls you will find in wp-admin / Classifieds / Payments History while editing a payment.
