apt - Cannot update and terminal says sources could not be read

$ sudo apt-get update
E: Type ' < !DOCTYPE' is not known on line 1 in source list /etc/apt/sources.list.d/medibuntu.list
E: The list of sources could not be read.

As the error points out, the /etc/apt/sources.list.d/medibuntu.list file is causing the issue. That file is used (in the past) to get list of packages maintained as part of Medibuntu. But, "Medibuntu has now been shut down, the packages were either obsolete, unnecessary or moved to the official Ubuntu archive.". Therefore, it is better to remove that file from your machine.

Run the following command to remove that file:

sudo rm /etc/apt/sources.list.d/medibuntu.list

Then run sudo apt-get udpate