<?php


use DrupalCoreFormFormStateInterface;
use Drupalcommerce_orderEntityOrder;
use SymfonyComponentHttpFoundationRedirectResponse;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function commerce_custom_form_commerce_order_item_add_to_cart_form_alter(&$form, FormStateInterface $form_state, $form_id) {
   $form['actions']['submit']['#value']='Sponsor';
   $form['#validate'][] = 'commerce_custom_form_validate';
   $form['actions']['submit']['#submit'][] = 'commerce_custom_cart_alter_redirect';
}

function commerce_custom_form_validate(array &$form, FormStateInterface $form_state) {

    $user = Drupal::currentUser()->id();
    $connection = Drupal::database();
    $query = $connection->query("select * from commerce_order_item join commerce_order on commerce_order_item.order_id = commerce_order.order_id where uid = :user and state = 'draft'", [':user' => $user]);
    $result = $query->fetchAll();
    //dpm($result);

    $cart_count = $result[0]->quantity;
    $order_id = $result[0]->order_id;

    if ($cart_count > 0 && $order_id) {
        $order = Drupal::entityTypeManager()->getStorage('commerce_order')->load($order_id);
        $items = $order->getItems();
        foreach($items as $item_key => $item) {
            $order->removeItem($item);
            $item->delete();
        }
    }
}

function commerce_custom_cart_alter_redirect(&$form, FormStateInterface $form_state) {
  $responce =  new RedirectResponse('/how-it-works');
  $responce->send();
}

<?php

namespace Drupalcommerce_customPluginBlock;
use DrupalCoreBlockBlockBase;

class ContinueCheckoutBlock extends BlockBase {

   public function build() {

        $user = Drupal::currentUser()->id();
        $connection = Drupal::database();
        $query = $connection->query("select * from commerce_order_item join commerce_order on commerce_order_item.order_id = commerce_order.order_id where uid = :user and state = 'draft'", [':user' => $user]);
        $result = $query->fetchAll();
        $order_id = $result[0]->order_id;

        return array(
            '#type' =>'markup',
            '#markup' => '<a href=checkout/' . $order_id . '/order_information class=button>Continue</a>',
        );
   }
}