For some reason I am getting an error message when I try to install opera-stable.
Here is output from my console.
I am not really too familiar with the way apt deals with the security keys but I wondered if it might be something wrong on the server.
$ sudo apt update
Hit:1 http://us.archive.ubuntu.com/ubuntu xenial InRelease
Get:2 http://us.archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]
Hit:3 http://ppa.launchpad.net/ondrej/php/ubuntu xenial InRelease
Get:4 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Hit:5 http://ppa.launchpad.net/webupd8team/java/ubuntu xenial InRelease
Get:6 http://us.archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]
Get:7 https://deb.opera.com/opera-stable stable InRelease [2,592 B]
Err:7 https://deb.opera.com/opera-stable stable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D615560BA5C7FF72
Fetched 309 kB in 1s (276 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
1 package can be upgraded. Run 'apt list --upgradable' to see it.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://deb.opera.com/opera-stable stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D615560BA5C7FF72
W: Failed to fetch https://deb.opera.com/opera-stable/dists/stable/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D615560BA5C7FF72
W: Some index files failed to download. They have been ignored, or old ones used instead.
In a terminal and in that order
sudo apt-key add archive.key
sudo apt-get update
and you are done