Ubuntu Doc page says this:
It is advisable that you add the Opera GPG key.
wget -qO - http://deb.opera.com/archive.key | sudo apt-key add -
Where do I add that?
I want to take the advice but I don't know what part of software center to add gpg keys to.
This a a one line command to enter in terminal. See What is a terminal and how do I open and use it?
To use it, you would paste the entire command in the terminal (remember to use https):
But of course, it is daunting just copying and pasting commands without knowing what they are doing, and having no instructions on how to undo their actions, so here is a basic breakdown of the commands:
wgetdownloads something from a server. See wget manual for Ubuntu 16.04.
|is a pipline, which takes the output of one command and runs it into the input of another
apt-key addadds a package key
So it basically downloads the key and then adds it in one command.
I tested the command and it should work.
Now to verify that it worked, run this command (from this answer):
This will list the keys added and the key from Opera should be listed on the bottom like this:
The linked answer also shows that you can remove the key if needed, using:
30C18A2Bbeing the key-id from the list.
After performing that command, and setting up the sources exactly like in your screen-shot, do:
(note there are some random warnings, but nothing that affects the install or software center operations)
And for the removal (just in case): What is the correct way to completely remove an application?
So in summary: