Ubuntu 20.04.3 LTS _Focal Fossa_ - Release amd64 (20210819)]/ focal main restricted

I have failed to install various programs on Ubuntu 20.04.3 LTS without knowing what's going on. Today I tried installing Zoom and got a clue about where the problem may be:

Malformed entry 53 in list file 

$ sudo apt update E: Malformed entry 53 in list file /etc/apt/sources.list (URI parse) E: The list of sources could not be read. $ sudo -H gedit /etc/apt/sources.list" 

I went in that file with: sudo -H gedit /etc/apt/sources.list expecting to erase a bad line (53), but what I found was a long list of messages describing a problem with the original installation and all the Ubuntu Repository Sources being restricted as result:

MAIN LIST FILE CONTENTS

# deb cdrom:[Ubuntu 20.04.3 LTS _Focal Fossa_ - Release amd64 (20210819)]/ focal main restricted # See for how to upgrade to # newer versions of the distribution. # deb-src focal main restricted ## Major bug fix updates produced after the final release of the ## distribution. # deb-src focal-updates main restricted ## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu ## team. Also, please note that software in universe WILL NOT receive any ## review or updates from the Ubuntu security team. # deb-src focal universe # deb-src focal-updates universe........................... 

*** Longer links list that I cant post because stack exchange limit of 10 ***

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