You will have the need to disable all payment gateways on the site, but just disabling them in WooCommerce > Settings > Payments will not work because then the customers will get an error message since there are no payment methods available.
We’re going to be using a different method here. The way to go is to add just a single line code to the functions.php file within your theme folder.
Here’s the code:
<?php // Do not include this if already open! Code goes in theme functions.php. add_filter( 'woocommerce_cart_needs_payment', '__return_false' );
The checkout page will now display only a Place Order button.