The pgAdmin 4 server could not be contacted error on Ubuntu 21.10

i installed pgAdmin but when i open it i get this error :

Failed to launch pgAdmin4. Error:
Error: spawn /usr/pgadmin4/venv/bin/python3 ENOENT

when i tap this command :

sudo python3 /usr/share/pgadmin4/web/pgAdmin4.py

i get

python3: can't open file '/usr/share/pgadmin4/web/pgAdmin4.py': [Errno 2] No such file or directory

then i create the missing file path as,

sudo mkdir -p /var/cache/pgadmin/sessions

and run this command again

sudo python3 /usr/share/pgadmin4/web/pgAdmin4.py

but i get the same error

Ran into this on Ubuntu 21.10 (impish), because I had borked my installation paths thinking pgadmin4 wasn't fully installed when it was.

The error is because it is assuming python3.8 is installed (21.10 comes with python3.9) , but it isn't, so all the symlinks and virtual env fails. But this is happening because you've installed a version of pgadmin4 that doesn't match your Ubuntu version.

garrett@garrett-external:/etc/apt/sources.list.d$ ls
deadsnakes-ubuntu-ppa-impish.list  pgadmin4.list.save  pgdg.list.save
pgadmin4.list                      pgdg.list


##Looking at pgadmin4.list.save , I see I had at least tried to use
# hirsute 's repo:

garrett@garrett-external:/etc/apt/sources.list.d$ cat pgadmin4.list.save
    deb https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/hirsute pgadmin4 main

##I'm thinking this is overriding the correct impish repos, so I'll
# try deleting that file, purging the installation and re-installing:

garrett@garrett-external:/etc/apt/sources.list.d$ ls
    deadsnakes-ubuntu-ppa-impish.list  pgadmin4.list  pgadmin4.list.save  pgdg.list  pgdg.list.save
garrett@garrett-external:/etc/apt/sources.list.d$ sudo rm pgadmin4.list.save

##double check that the pgadmin4 sources are correct...
garrett@garrett-external:/etc/apt/sources.list.d$ ls
    deadsnakes-ubuntu-ppa-impish.list  pgadmin4.list  pgdg.list  pgdg.list.save
garrett@garrett-external:/etc/apt/sources.list.d$ cat pgadmin4.list
  deb https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/focal pgadmin4 main

##^ok definitely the problem...should fix that....

garrett@garrett-external:/etc/apt/sources.list.d$ sudo sed -i 's/focal/impish/g' /etc/apt/sources.list.d/pgadmin4.list
garrett@garrett-external:/etc/apt/sources.list.d$ cat ./pgadmin4.list
  deb https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/impish pgadmin4 main

##uninstall pgadmin4
garrett@garrett-external:/etc/apt/sources.list.d$ sudo apt purge pgadmin4
garrett@garrett-external:/etc/apt/sources.list.d$ sudo apt autoremove

##reinstall pgadmin4
garrett@garrett-external:/etc/apt/sources.list.d$ sudo apt update && sudo apt install pgadmin4

And now it works fine

EDIT: DON'T GO DOWN THE RABBIT HOLE BELOW

Workaround: Manually install python3.8 using the deadsnakes PPA and some temporary apt cache updating trickery

  #first add deadsnakes ppa...
  $  sudo add-apt-repository ppa:deadsnakes

  # Following the gist of https://brennan.io/2021/06/21/deadsnakes-hirsute/ ,
  # update the deadsnakes ppa's app repos to look at the past 2 LTS versions
  $ sudo vim /etc/apt/sources.list.d/deadsnakes-ubuntu-ppa-impish.list
     ---> change 
      'deb http://ppa.launchpad.net/deadsnakes/ppa/ubuntu/ impish main'  to 
      'deb http://ppa.launchpad.net/deadsnakes/ppa/ubuntu/ bionic main' , also add 
      'deb http://ppa.launchpad.net/deadsnakes/ppa/ubuntu/ focal main'


   #Add the following rules which blacklist all packages from 
   # the repos, and then selectively enable Python versions from the correct
   # locations. This can be put in any filename within the directory
   #      /etc/apt/preferences.d/  (I named mine deadsnakes_pref)

Explanation: Prevent installing from deadsnakes repo.
Package: *
Pin: release o=LP-PPA-deadsnakes
Pin-Priority: 1

Explanation: Allow installing python 3.{6,7} from deadsnakes/focal
Package: *python3.6* *python3.7*
Pin: release o=LP-PPA-deadsnakes,n=focal
Pin-Priority: 500

Explanation: Allow installing python 3.8 from deadsnakes/bionic
Package: *python3.8*
Pin: release o=LP-PPA-deadsnakes,n=bionic
Pin-Priority: 500

 ##temporarily change all your sources to bionic, so the python3.8 
 #dependencies will be met
   $  sudo sed -i 's/impish/bionic/g' /etc/apt/sources.list

  ## try to update the apt cache and install python3.8 . You'll get yelled
  #  at for not having the proper key ring:
  ****************
W: GPG error: http://us.archive.ubuntu.com/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://us.archive.ubuntu.com/ubuntu bionic InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
****************

## so following 
## https://askubuntu.com/questions/13065/how-do-i-fix-the-gpg-error-no-pubkey  
## , I add the key
$  sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3B4FE6ACC0B21F32

# now this should work
$ sudo apt update && sudo apt install python3.8

#after we're done, don't forget to change back your sources
$ sudo sed -i 's/bionic/impish/g' /etc/apt/sources.list
$ sudo apt update

#And now pgadmin4 should work. If you want to run it from the terminal, 
# add this line to your ~/.bash_aliases  file:
   alias pgadmin4='/usr/pgadmin4/bin/pgadmin4'  

Just install the Pgadmin again and it will fix the broken links after the OS upgrade ;)

If you want instructions, you can follow the ones from here: https://computingforgeeks.com/how-to-install-pgadmin-4-on-ubuntu/