I managed to duplicate the issue on your checkout page. The error notice appears to be showing on the side rather than appearing at the top as it should.
This kind of problem is usually caused by a conflict with your theme or with another plugin. The best way to determine this is to:
Temporarily switch your theme to Storefront or Twenty Twenty-Four
Disable all plugins except for WooCommerce
Repeat the action that is causing the problem
If you’re not seeing the same problem after completing the conflict test, then you know the problem was with the plugins and/or theme you deactivated. To figure out which plugin is causing the problem, reactivate your other plugins one by one, testing after each, until you find the one causing conflict. You can find a more detailed explanation on how to do a conflict test here.
I hope that helps! Let me know once you have done a conflict test!