Do you see this error when you are trying submit Checkout page?

Please open your wp-config.php file, find

define( 'WP_DEBUG', false );

and change it to

define( 'WP_DEBUG', true );

Now, if you submit the checkout page again – you should see the exact error message. E.g. this one:

Please try to deactivate your plugins one-by-one to find which one causes this error.