Some auction website owners need only bidding system and payment is sorted offline so they do now want to display pay now button once auction has finished.
To do that you need to edit two templates in your child theme, templates\single-product\pay.php (remove all lines except if with // Exit if accessed directly ) and templates\loop\pay-button.php – also remove all except that if statement.
Templates should go to your-child-theme\woocommerce\single-product\pay.php and your-child-theme\woocommerce\loop\pay-button.php
There is also pay button if user watches auction and ajax check is enabled, this one needs to be hidden using CSS:
.product-type-auction a.button {display: none}