The drupal/console package was never made fully compatible with Drupal 9, and its development has now stopped. If you are starting a recent project, the command-line reference is now Drush.
The modern solution: Drush
Drush is actively maintained and compatible with Drupal 9, 10 and beyond:
composer require drush/drush
You then access the commands through vendor/bin/drush: code generation, cache management, migrations, and so on.
If you really need drupal/console
For an existing project that still depends on it, installing on Drupal 9 requires a few options passed to Composer to force resolution:
composer require drupal/console:~1.0 --prefer-dist --optimize-autoloader --sort-packages --no-update composer update
Keep in mind that the tool is no longer maintained: this is a workaround, not a long-term base. The original issue is in fact still open.