Drupal claims that updating sites from D8 to D9 is easy. That is true if you have no additional modules installed. But in the real world, it is really difficult. I have done 4 sites now, and all were somewhat different, alas. The issue is updating the modules, or removing those that cannot be updated to D9. Here is my procedure.
Make sure your D8 site is fully updated
Update everything to the latest Drupal8 (currently 8.9.6) version.