I managed to get rid of that doing

curl -fsSL https://deb.opera.com/archive.key | gpg --dearmor | sudo tee /usr/share/keyrings/opera.gpg > /dev/null

and

echo deb [arch=amd64 signed-by=/usr/share/keyrings/opera.gpg] https://deb.opera.com/opera-stable/ stable non-free | sudo tee /etc/apt/sources.list.d/opera.list

In general I followed this blog post.

Hope this helps.