That is expected, since there is no file or folder "/etc/apt/sources.list". Forget everything you may have learned from that outdated 2022 guide.sudo nano /etc/apt/sources.list did bring the text editor, (labeled 'GNU nano 8.4'), but it was blank, no list.
There is a folder named "/etc/apt/sources.list.d". It contains various text files that end with ".sources". That's what Trixie is now using.
You may find it easier to boot a Live USB of a distro that would allow you to edit those files on the Debian install partition with a familiar GUI text editor. Remember, these are system files, and must be edited as system admin--with sudo or as root.
Statistics: Posted by stevepusser — 2025-12-29 16:24