Can't Install: Unmet dependencies

It seems like this module has a dependency on another module named su_events, but after searching around I could not find a module with this name. Is that a custom module that is only used internally within Students' Union UCL?

It is, apologies. That should have been removed before it was published. I'll release a fix in the next few days.

@maxwellkeeble after further testing, there is also an unnecessary dependency on commerce_stock which prevents installation on fresh instances of Drupal Commerce sites;
The class \Drupal\commerce_cart_webform\EventSubscriber\OrderEventSubscriber uses the trait \Drupal\commerce_stock\ContextCreatorTrait but never calls any methods implemented by the trait.

All the phpdoc of the methods also imply that they are doing something with the product stock, even though they don't.

    if ($this->variation_webform == NULL) {
      // $purchased_entity_id may be null causing null ID error when trying to load the webform.
      $purchased_entity_id = $form_state->getValue('purchased_entity')[0]['variation'];
      $variation = ProductVariation::load($purchased_entity_id);
      $variation_webform = $this->getVariationCartWebform($variation);
      $this->variation_webform = $variation_webform;
    }

Suggestion

Add a check for existence of the purchased_entity submitted value and use the order item's default purchased entity as a fallback.

    if ($this->variation_webform == NULL) {
      if ($form_state->hasValue('purchased_entity')) {
        $purchased_entity_id = $form_state->getValue('purchased_entity')[0]['variation'];
        $variation = ProductVariation::load($purchased_entity_id);
      }
      else {
        $order_item = $this->entity;
        $variation = $order_item->getPurchasedEntity();
      }
      $variation_webform = $this->getVariationCartWebform($variation);
      $this->variation_webform = $variation_webform;
    }