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
and change to