Let’s take a simple example of running the command deb as mentioned here.

$ deb http://deb.opera.com/opera/ stable non-free

No command 'deb' found, did you mean:

 Command 'debc' from package 'devscripts' (main)

 Command 'derb' from package 'libicu-dev' (main)

 Command 'dab' from package 'bsdgames' (universe)

 Command 'debi' from package 'devscripts' (main)

deb: command not found

If you have come across this mistake/error, then you have probably done a common mistake. Whenever you see any line starting with deb, make sure that it is not for running in the command prompt, but for adding into a particular file

The line must be added to the file /etc/apt/sources.list

Just like the example on how to install Opera in Linux, the line must be added to /etc/apt/sources.list.

After this, you can install the concerned software in your machine using apt-get.