You need to check third-party repos, looks like mod-pagespeed repo broke apt update. https://support.plesk.com/hc/en-us/articles/12377081728407-Unable-to-update-system-packages-on-Plesk-server-The-repository-http-dl-google-com-linux-mod-pagespeed-deb-dists-stable-Release-gpg-is-not-signed