Err:1 https://deb.opera.com/opera-stable stable/non-free amd64 opera-stable amd64 136.0.6008.52
  404  Not Found [IP: 95.101.23.81 443]
Error: Failed to fetch https://deb.opera.com/opera-stable/pool/non-free/o/opera-stable/opera-stable_136.0.6008.52_amd64.deb  404  Not Found [IP: 95.101.23.81 443]
Error: Unable to fetch some archives, maybe run apt update or try with --fix-missing?

The installed version is opera://version/
Opera 136.0.6008.52 (Official Build) Stable (x86_64)

See https://deb.opera.com/opera-stable/pool/non-free/o/opera-stable/
The current package listed there is the outdated → stable_135.0.5973.142_amd64.deb

Via https://download.opera.com/download/get/?partner=www&opsys=Linux it’s the opera-stable_136.0.6008.52_amd64.deb

And yes, the constant error messages about the outdated opera-stable.list instead of the current opera-stable.sources are annoying and embarrassing.