I'm so sorry if I couldn't be more helpful to you. But here what I did in the past where I faced an issue that were caused by the repositories. The outputs were these:
W: GPG error: http://download.opensuse.org/repositories/home:/ungoogled_chromium/Ubuntu_Focal InRelease: The following signatures were invalid: EXPKEYSIG 02456C79B2FD48BF home:ungoogled_chromium OBS Project <home:ungoogled_chromium@build.opensuse.org>
E: The repository 'http://download.opensuse.org/repositories/home:/ungoogled_chromium/Ubuntu_Focal InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Then I opened up Software & Updates,
Choose Other Software
Then scrolled down until I found http://download.opensuse.org/repositories/home:/ungoogled_chromium/Ubuntu_Focal InRelease: and unticked it, and then press Closeand proceed to Reload
After that, I tried sudo apt update again and everything is back to normal.
Now, it's important to note that I don't suggest you to do this if the terminal outputs doesn't specify which repository that caused a problem.