Comment 11 for bug 1885644

Yes we're not offering the update to users of 20.04.2 because bugs in the version of subiquity in the ISO make the upgrade exceedingly bumpy :( But if you're autoinstalling, you can set the installer to refresh from stable with:

refresh-installer:
  channel: stable
  update: true

Or you can just use an ISO from http://cdimage.ubuntu.com/ubuntu-server/focal/daily-live/current/ if you don't mind using an ISO based on the state of the archive on a random day vs release day (in practice it's not going to make a lot of difference)