"Bulk Update" column broken when grouping due to wrong "use" statement.

The location of that plugin was apparently changed in Drupal core 8.5. See the change record: Drupal\system\Plugin\views\field\BulkForm moved to the Views module →
The old location was deprecated but still worked until it was finally removed in January 2020.

It seems that no-one has been using this feature in the views_aggregator module, because this has been broken for three years!

Thanks for finding the problem. Committed patch #2.

Automatically closed - issue fixed for 2 weeks with no activity.