Ubuntu 18.04 - Fix unable to locate package ***

1.
Open the file /etc/apt/sources.list

2.
Add the following lines:

deb http://archive.ubuntu.com/ubuntu bionic multiverse
deb http://archive.ubuntu.com/ubuntu bionic-security multiverse
deb http://archive.ubuntu.com/ubuntu bionic-updates multiverse
deb http://archive.ubuntu.com/ubuntu bionic universe
deb http://archive.ubuntu.com/ubuntu bionic-security universe
deb http://archive.ubuntu.com/ubuntu bionic-updates universe

1.
Save the file.

2.
sudo apt-get update