Error: Class "Drupal\webform\Commands\WebformCliService" not found

The WebformCliService was removed via #3220309: [Drupal 9.x] Only support Drush 10 for Drupal 9.x+ and Webform 6.x+ → .

A few options to figure out what is wrong

• Clear your cache
• Search your code base for WebformCliService and see if you have an old version of the webform module.
• Delete and reinstall the webform module
• Rebuild your code base by deleting the module/contrib folder and rerunning composer install

Additionally I would check on any patches that may be installed and any webform-related contributed or custom modules.