/etc/apt/sources.list" E212: Can't open file for writing

For some reason the file you are writing to cannot be created or overwritten.
The reason could be that you do not have permission to write in the directory
or the file name is not valid.

Vim has a builtin help system. I just quoted what it says to :h E212.

You might want to edit the file as a superuser as sudo vim FILE. Or if you don't want to leave your existing vim session (and now have proper sudo rights), you can issue:

:w !sudo tee % > /dev/null

Which will save the file.

HTH

That happens to me all the time, I open a root file for writing:

Instead of losing all your changes and re-opening with sudo. See this demo of how to save those changes:

One time Setup demo to create a root owned read only file for a lower user:

sudo touch temp.txt
sudo chown root:root temp.txt
sudo chmod 775 temp.txt
whoami
el

First open the file as normal user:

vi temp.txt

Then make some changes to the file, it warns you its read only. Use this command.

:w !chmod 777 %

Then write the file:

:wq!

The permissions are expanded, and the file is saved. You need the exclamation point because you are editing a root file as a lesser user.

Explanation of what that command does:

The :w means write the file. The bang means start interpreting as shell. chmod means change permissions, 777 means full permissions everywhere. The percent means the current file name.

It applies the change. And it ask if you want to re-load. Press "O" for "Ok". Don't reload or you'll lose your changes.

For me there was was quite a simple solution. I was trying to edit/create a file in a folder that didn't exist. As I was already in the folder I was trying to edit/create a file in.

i.e. pwd folder/file

and was typing

sudo vim folder/file

and rather obviously it was looking for the folder in the folder and failing to save.