Hi @xavi74!
I understand how frustrating this can be, but rest assured, we will work with you until the issue is solved.
I visited your site and replicated the issue. I also checked your system status report; I can see that you are using Elementor and various plugins to add features to Elementor.
The cash on delivery purchase has worked correctly, but what the customer is asking for is a credit card purchase, which is what is giving me an error.
Since Cash on Delivery is a core WooCommerce payment option, and it is working fine, different from credit cards (you need a plugin to use credit cards like WooPayments), which are the ones giving trouble, I think a conflict between WooCommerce, WooPayments and your theme or another plugin installed might be happening in your site.
To be sure, please perform a conflict test by temporarily switching your theme back to Storefront, deactivating all plugins except WooCommerce and WooPayments, and then checking again if the issue persists.
If that solves it, reactivate the theme first and check again. Then, reactivate your other plugins, testing after each until you find the one causing conflict.
If you cannot perform a conflict test on your live site, we recommend creating a staging site using the WP Staging plugin.
You can find a more detailed explanation of how to do a conflict test here: https://woo.com/document/how-to-test-for-conflicts/.
Let us know how that goes for you!