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) and possibly newer releases.
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)