You might have run into this error when updating packages. The error is probably because you don’t have the public key corresponding to whatever private key the package was signed with. Hopefully, you can just download the public key from keyserver.ubuntu.com and get on with your day. In my case, it was a Google SDK key
8B57C5C2836F4BEB that was missing. Just use the following command, replacing the key ID with the key you need.
sudo apt-get update should just work. If it doesn’t, feel free to leave a comment.