Installing packages in Ubuntu R: after editing my /etc/apt/sources.list file, apt-get update doesn't work anymore

[mara]

@Andrea

W: GPG error: https://cloud.r-project.org/bin/linux/ubuntu xenial/ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9 
E: The repository 'https://cloud.r-project.org/bin/linux/ubuntu xenial/ InRelease' is not signed. 
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'm not running Ubuntu, but it looks like it's a security issue that you'll have to configure for your machine.

Couple possibly relevant threads:

[SOLVED] Multiple issues with "sudo apt-get update"

Data from such a repository can't be authenticated