Hi everyone. I’m very new to Linux; earlier today I successfully loaded Mint onto a Dell Latitude laptop, wiping Win10 at the same time. Everything was going along very nicely (successfully added Brave and a password manager) until I apparently did something stupid. During one boot sequence, I was presented with the option of checking a box to allow the CDROM image created during the Mint download/setup process to be used as a repository. Not sure why I checked that box 
Now when I try to install software like Nordvpn or Signal, I get this message:
E: The repository ‘cdrom://Linux Mint 20.2 Uma - Release amd64 20210703 focal Release’ does not have a Release file.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
I’ve searched for info on modifying the repository list and found articles suggesting I use the command:
$ sudo nano /etc/apt/sources.list
That brings up a list edit type screen in which only one repository is shown, and it’s the CDROM. Following the instructions on the Linux tech article, I would delete this repository from this list and save it, then update and reboot, but since I have NO CLUE if I’m in the right zip code, and since there are no other repositories shown, I’m afraid to do that not knowing what the fallout will be.
Here’s what that edit screen shows after entering the sudo nano /etc/apt/sources.list command. I think the repository is the line at the top beginning with “deb”
deb cdrom:[Linux Mint 20.2 Uma - Release amd64 20210703]/ focal contrib main
This system was installed using small removable media
(e.g. netinst, live or single CD). The matching “deb cdrom”
entries were disabled at the end of the installation process.
For information about how to configure apt package sources,
see the sources.list(5) manual.
Should I delete the line “deb cdrom…” and reboot, or something else entirely? Thanks!
