How can i stop woocommerce checkout event based on api request result?

Add this as a validation in the checkout

Something like this:

add_action( 'woocommerce_after_checkout_validation', 'my_validation', 9999, 2);

function my_validation( $fields, $errors ){

    if([your validation]){
        return;
    }else{
        $errors->add( 'stock', 'your error message' );
    }
}