I am using Ubuntu 16.04 LTS.
Whenever I try to install updates using the terminal window, I get this error.
Also when I go into the Software Center there is software shown.
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Sudo is working fine, you my guess is you edited
/etc/apt/sources.list and have a typo.
Open an editor an comment out line 54
sudo nano /etc/apt/sources.list
Find line 54 and remove it
See also How to show line numbering in nano when opening a file for information on how to show and go to specific lines with nano.
If you need help, post the contents of the file, better line 54.