Command "generate-votes" is not defined
This command doesn't come from Drush core but from the Voting API module. This command simply doesn't exist yet in the currently latest Voting API release: 8.x-3.0-beta2.  See, the directory /src/Commands where Drush commands since Drush version 9 get registered now simply isn't there yet.
You need to get the latest Voting API dev:
$ composer require 'drupal/votingapi:3.x-dev@dev'

There the /src/Commands directory contains a VotingApiCommands.php file with all the commands in it. The module needs to be enabled to use them, of course.

Voting API version 3.0-beta2 only contains the legacy votingapi.drush.inc file for Drush commands up to Drush version 8.