Mrthdrb

Working on Magento 2 checkout validation following is the code

<?php
namespace VendorNameModuleNameBlock;

class LayoutProcessor

 /**
 * @param MagentoCheckoutBlockCheckoutLayoutProcessor $subject
 * @param array $jsLayout
 * @return array
 */
 public function afterProcess(
 MagentoCheckoutBlockCheckoutLayoutProcessor $subject,
 array $jsLayout
 ) 
 $jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']['children']
 ['shippingAddress']['children']['shipping-address-fieldset']['children']['street'] = [
 'component' => 'Magento_Ui/js/form/components/group',
 'label' => __('Address'),
 'required' => true,
 'dataScope' => 'shippingAddress.street',
 'provider' => 'checkoutProvider',
 'sortOrder' => 60,
 'type' => 'group',
 'children' => [
 [
 'component' => 'Magento_Ui/js/form/element/abstract',
 'config' => [
 'customScope' => 'shippingAddress',
 'template' => 'ui/form/field',
 'elementTmpl' => 'ui/form/element/input'
 ],
 'dataScope' => '0',
 'provider' => 'checkoutProvider',
 'validation' => true,
 'additionalClasses' => 'additional',
 'label' => __('Street')
 ],
 [
 'component' => 'Magento_Ui/js/form/element/abstract',
 'config' => [
 'customScope' => 'shippingAddress',
 'template' => 'ui/form/field',
 'elementTmpl' => 'ui/form/element/input'
 ],
 'dataScope' => '1',
 'provider' => 'checkoutProvider',
 'validation' => true,
 'label' => __('Number')
 ],
 [
 'component' => 'Magento_Ui/js/form/element/abstract',
 'config' => [
 'customScope' => 'shippingAddress',
 'template' => 'ui/form/field',
 'elementTmpl' => 'ui/form/element/input'
 ],
 'dataScope' => '2',
 'provider' => 'checkoutProvider',
 'validation' => true,
 'label' => __('District')
 ]
 ]

 ];

 return $jsLayout;
 
 

How to change the vallidation message for the fields Please help

You can change validation message via magento translations.

More information here http://devdocs.magento.com/guides/v2.1/frontend-dev-guide/translations/xlate.html

Here is ways how to add translations:

- add new translation to theme

app/design/frontend/Vendor/theme/i18n/en_US.csv (low priority: 3)

More information here http://devdocs.magento.com/guides/v2.1/frontend-dev-guide/themes/theme-create.html

- create your own language package

app/code/Vendor/MyLocale/en_US.csv (priority: 2)

More information here http://devdocs.magento.com/guides/v2.0/config-guide/cli/config-cli-subcommands-i18n.html#config-cli-subcommands-xlate-pack

- use Inline Translation in admin (high priority: 1)

Priority defines which translation dictionary will loaded first.

JS translations stored in static files so you need to re-deploy static content. Or you can just remove js-translation.json files. In magento root folder run

find pub/static/frontend -name js-translation.json -exec rm -rf ;

You should be able to translate the error messages via i18n package.
But there is a bug in Magento up to 2.1.7 at least that won't let you translate these error messages.
You can read this thread for possible solutions. Although I tried all of them and for me none worked so far.

https://github.com/magento/magento2/issues/4883