PgAdmin: Package 'pgAdmin4' has no installation candidate

Here's how I solved the issue;

The issue arises because the repository for pgadmin has not been added to your local machine. To solve the issue, do the following below;

Note: This should work in Ubuntu 16.04 (Xenial), Ubuntu 18.04 (Bionic), Ubuntu 19.10 (Eoan), Ubuntu 20.04 (Focal), Debian 9 (Stretch) and Debian 10 (Buster)

Install the public key for the repository (if not done previously):

curl https://www.pgadmin.org/static/packages_pgadmin_org.pub | sudo apt-key add

Create the repository configuration file:

sudo sh -c 'echo "deb https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/$(lsb_release -cs) pgadmin4 main" > /etc/apt/sources.list.d/pgadmin4.list && apt update'

Choose the Installation method that you want:

• Install pgAdmin (for both desktop and web modes):

  sudo apt install pgadmin4
  

• Install pgAdmin (for desktop mode only):

  sudo apt install pgadmin4-desktop
  

• Install pgAdmin (for web mode only):

  sudo apt install pgadmin4-web 
  

You can configure the webserver, if you installed pgadmin for web mode:

sudo /usr/pgadmin4/bin/setup-web.sh

Resources: pgAdmin 4 (APT)

That's all.

I hope this helps.

In my case, the following is working:

1st step:

curl https://www.pgadmin.org/static/packages_pgadmin_org.pub | sudo apt-key add

Create the repository configuration file. In the above answer, the command mentioned below is now too old and the page returns 404, so the packages will not be downloaded successfully.

sudo sh -c 'echo "deb https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/$(lsb_release -cs) pgadmin4 main" > /etc/apt/sources.list.d/pgadmin4.list && apt update'

2nd step:

The updated and working command is:

sudo sh -c 'echo "deb https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/focal/ pgadmin4 main" > /etc/apt/sources.list.d/pgadmin4.list && apt update'

3rd step:

sudo apt install pgadmin4

For Ubuntu 18.04 you have to add the Postgresql repository and key to apt:

wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
 sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ `lsb_release -cs`-pgdg main" >> /etc/apt/sources.list.d/pgdg.list'

After this you have to run

sudo apt update

Then you can install, ex. with "sudo apt install pgadmin4 pgadmin4-apache2 -y"