When upgrading to Drupal 9 you can encounter the following issue: 


In Container.php line 151:

  You have requested a non-existent service "entity.manager".

Solution: look up every appearance of "entity.manager" in your code. The module probably has a newer version for Drupal 9 and older. 

I had to upgrade drupal/webform to ^6.0.0.

Or look in code for deprecaded

 $container->get('entity.manager')

and change to

 $container->get('entity_type.manager')

Need more help upgrading?

https://stefvanlooveren.me/blog/how-upgrade-drupal-8-9-composer