Generating Ubercart orders programmatically
I was able to resolve this by adding these to my file:

//for order
use Drupal\Core\Form\FormBase;
use Drupal\uc_order\Entity\Order;
use Drupal\user\Entity\User;
//for products
use Drupal\uc_order\EditableOrderPanePluginBase;
use Drupal\uc_order\Entity\OrderProduct;
use Drupal\uc_order\OrderInterface;