You can only edit /etc/apt/sources.list as root, so open the application to edit the file using sudo in front, eg 'sudo vi /etc/apt/sources.list' (without the quotes) Or replace vi with any other text editor you prefer.
You can only edit /etc/apt/ sources. list as root, so open the application to edit the file using sudo in front, eg 'sudo vi /etc/apt/ sources. list' (without the quotes) Or replace vi with any other text editor you prefer.