I just had the same problem.
The above command did not work at all. But upon inspection of the sources.list file, I saw a line that didn't belong there. I remember trying to add a custom package path. (it didn't work btw) This manual adding of a location was done within the package manager itself. This method cant be used again since the package manager doesn't launch any more because of the line I added. Trying to remove the line using text editor did not work either. The file would only open in read only mode.
Run terminal as root or super user.
(YOU ARE ABOUT TO USE A VERY POWERFUL TOOL IN SUPERUSER MODE. BE VERY CAREFUL)
type:
vi /etc/apt/sources.list
This should open a text editor within the terminal
use arrow keys to navigate to the invalid line.
ALL VALID LINED SHOULD START WITH DEB
on your keyboard tap dd while the cursor is in the invalid line (the d key x2)
NOWHERE ELSE
If the line is successfully removed, hold down shift and tap zz (the z key x2)
Rather exit the terminal and call a professional if something goes wrong.