Cannot install software: "Updating from such a repository can't be done securely"

Am trying to install Signal by Open Whisper Systems (a very reputable company and software) and I got the following error message. How can I continue the installation and will it be safe? Thanks.

Usually, the message saying that the Release file is missing means that the repository doesn’t provide packages for your version of Ubuntu.

In that case, though, the repo is the 16.04.3 CD-Rom that’s set in your sources. Either the disc is not in your drive and the system can’t find it, or you’re on 17.04/17.10 and the CD-Rom doesn’t match.

In any case, I’d suggest that you disable/remove the CD-Rom from your sources list, it’s useless to have it unless you don’t have an internet connection and need to install packages that are on it.

I really do need the packages that are on it though hmmm

You need the packages that are on the CD-Rom? Don’t you have a constant internet connection?

Does the error message appear when the disc is in the drive? If so, maybe the name of the disc is wrong for some reason. Try to remove it from your sources, then insert the disc and it should offer to add it again.