I am running Linux Mint 20 Cinnamon, too. I found this thread a week, or so ago for the same issue. I deleted the opera-stable.postinst file then and the issue was abated.
I just got an update for Opera and after installing it the error reappeared. So, I deleted the opera-stable.postinst file again.
Any information on actually fixing the actual issue? Is it Opera, LM20, dpkg, or what?
Thanks,