With three days to go until Drupal 8's end of life (on November 2, 2021), now is a good time to take stock of your Drupal 8 sites' modules. Use Upgrade Status to check for environment and module compatibility with Drupal 9.
Unless you are on Drupal 8.8.x or Drupal 8.9.x, Upgrade Status will tell you to move to that version first, before you can upgrade to Drupal 9. (At this point it should really tell you that 8.9.x is the only acceptable version to upgrade to). Why is that?