How to fix E: The list of sources could not be read in APT

Run the diagnostic command:

sudo apt-get check

This will output which file contains the malformed entry, for example:

E: Malformed entry 1 in sources file /etc/apt/sources.list.d/rethinkdb.list (Component)
E: The list of sources could not be read

Note the file path and line number mentioned in the error.

Use a text editor to open the file identified in the previous step:

sudo nano /etc/apt/sources.list.d/rethinkdb.list

If the error is in the main sources list, edit:

sudo nano /etc/apt/sources.list

Look at the line number mentioned in the error. Check for:

- Missing or extra spaces
- Incomplete repository URL
- Invalid component name
- Missing distribution codename

A valid source line looks like:

deb https://archive.ubuntu.com/ubuntu/ bionic main universe multiverse

Either fix the syntax or delete the entire line if you don't recognize the repository.

Save the file (in nano: Ctrl+O, then Ctrl+X).

Run apt-get check again:

sudo apt-get check

If the error is gone, continue to the next step. If the same file is still reported, check for additional broken lines.

Now rebuild the package cache with:

sudo apt update

This downloads fresh package lists from all configured sources. The error should not appear.

If the file contains only a third-party repository you no longer need, remove it entirely:

sudo rm /etc/apt/sources.list.d/rethinkdb.list
sudo apt update

Do NOT remove /etc/apt/sources.list itself, as it contains the main Ubuntu/Debian repositories. Only remove files in the .d/ directory if they are causing issues.