mysql – “An error occurred during the signature verification.” in apt-update after adding key 3A79BD29 and sudo apt-key del on Debian

the server I work on is a Debian 9.13 OS and “mysql Ver 14.14 Distrib 5.7.34,”. I tried downloading package information from all configured sources with the following command: sudo apt update then I faced to the following error: Hit:1 http://security.debian.org stretch/updates InRelease Hit:2 http://packages.cloud.google.com/apt google-cloud-monitoring-stretch-all InRelease Hit:3 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease Hit:4 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease … Read more