If for some reason your commerce site gets stuck while in development with some bad data for user cart, to get rid of all the carts you can fire this code to delete all the carts for that user (probably admin or user 1)
$user_id = \Drupal::currentUser()->id();
$carts = \Drupal::entityTypeManager()
->getStorage('commerce_order')
->loadByProperties(['uid' => $user_id, 'cart' => '1']);
foreach ($carts as $cart) {
$cart->delete();
}