Hello
I had the same problem in my Ubuntu.
I solved it like this: first I downloaded the updated key with the command:

sudo wget -qO- https://deb.opera.com/archive.key | gpg --dearmor | sudo dd of=/usr/share/keyrings/opera-browser.gpg

Then I corrected the file /etc/apt/sources.list.d/opera-stable.list to this state:

deb [signed-by=/usr/share/keyrings/opera-browser.gpg] https://deb.opera.com/opera-stable/ stable non-free

Now everything is updating normally for me.