The warnings are gone!

I have now found the following page with advice on how to add a user to /etc/sudoers: http://www.ghacks.net/2010/01/06/how-to-add-users-to-etcsudoers/

After making the recommended change to the file I ran

wget -qO- https://deb.opera.com/archive.key | sudo apt-key add -

and got an "OK". Let's see if this resolves the original problem.

The command gave the following:

magnus@ASUS-AMD:~$ wget -qO- https://deb.opera.com/archive.key | sudo apt-key     add -
[sudo] password for magnus: 
magnus finns inte i filen sudoers.  Denna incident kommer att rapporteras.

which means that my user account magnus is not in the file sudoers and that the incident will be reported. I have not found out how to add my account to the file sudoers but I guess it involves editing /etc/sudoers. Further assitance is needed.

Thanks, Red-wizard. Today, before trying out your terminal command I got the following: E: http://deb.opera.com stable InRelease is not (yet) available ()

Here is the warning message: "W: GPG-fel: http://deb.opera.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 63F7D4AFF6D61D45"

When using Opera 12.16 as an e-mail client in Debian 8.5 I often get security warnings about a GPG key that couldn't be verified. What should I do?