Add a button to view and add submit handler -taken from Commerce2.x D8

view_submit_button.txt

/**
 * Implements hook_form_alter().
 */
function commerce_checkout_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if (strpos($form_id, 'views_form_commerce_cart_form_') === 0) {
    // Only add the Checkout button if the cart form view has order items.
    $view = reset($form_state->getBuildInfo()['args']);
    if (!empty($view->result)) {
      $form['actions']['checkout'] = [
        '#type' => 'submit',
        '#value' => t('Checkout'),
        '#weight' => 5,
        '#access' => \Drupal::currentUser()->hasPermission('access checkout'),
        '#submit' => array_merge($form['#submit'], ['commerce_checkout_order_item_views_form_submit']),
        '#order_id' => $view->argument['order_id']->value[0],
      ];
    }
  }
}

/**
 * Submit handler used to redirect to the checkout page.
 */
function commerce_checkout_order_item_views_form_submit($form, FormStateInterface $form_state) {
  $order_id = $form_state->getTriggeringElement()['#order_id'];
  $form_state->setRedirect('commerce_checkout.form', ['commerce_order' => $order_id]);
}