How to call User::load with dependency injection
You should use entity_type_manage to get the user storage. Something like this:  

&lt;?php

namespace Drupal\country_selector\Plugin\Block;

use Drupal\Core\Session\AccountProxyInterface;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\user\UserStorageInterface;
/**
 * Provides a 'DefaultBlock' block.
 *
 * @Block(
 *  id = "default_block",
 *  admin_label = @Translation("Country switcher"),
 * )
 */
class DefaultBlock extends BlockBase implements ContainerFactoryPluginInterface {

  protected $account;
  protected $userStorage;

  /**
   * Class constructor.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, AccountProxyInterface $account, UserStorageInterface $user_storage) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this-&gt;account = $account;
    $this-&gt;userStorage = $user_storage;
  }

  /**
   * {@inheritdoc}
  */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container-&gt;get('current_user'),
      $container-&gt;get('entity_type.manager')-&gt;getStorage('user')
    );
  }

  /**
   * {@inheritdoc}
  */
  public function build() {
    $build = [];

    $userIdx = $this-&gt;$acount-&gt;id();
    $userx = $this-&gt;userStorage-&gt;load($userIdx);
    $countryx = $userx-&gt;country_iso_code_2;
    .....


Hope this helps.