If you are looking to update to install apps, then I have a workaround (see below).
Currently upgrading breaks the installation, which appears to be due to critical gemian apps being downgraded with update.
Working out which Gemian packages to hold prior to an upgrade is on my to do list.
UPDATE: See comment from Shymega below - he is concerned about the risks of this approach, which is understandable. Note: this is currently the only way to install apps. I recommend only installing apps that you know are going to be ok with this install method (see my install guide) or only installing apps on a Cosmo that you don't mind re-formatting if things going wrong.
I recommend uncommenting the line again afterwards to avoid accidentally upgrading anything (which WILL break your installation).
See my linux installation guide for the Communicator:
https://www.oesf.org/forum/index.php?topic=36822.msg298717#msg298717----
#REPOSITORIES
#unfortunately deb
https://gemian-planet.thinkglobally.org/buster/ buster main has been taken down by Planet, so we need to comment it out in the sources list.
#vi can be flaky and we can't install vim yet, so use nano to edit the file
sudo nano /etc/apt/sources.list.d/gemian.list
#comment out the currently offline old V3 firmware planet repository
deb https://gemian.thinkglobally.org/buster/ buster main
#deb https://gemian-planet.thinkglobally.org/buster/ buster main
#save the file and exit
sudo apt update
#when asked about accepting that the repository has changed from stable to oldstable, select 'yes'
# IMPORTANT: DO NOT RUN sudo atp upgrade AT ANY TIME - THIS WILL BREAK THE INSTALLATION