Beginners • Re: I'm a beginner and I can't use sudo apt-get install anymore! Help!

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W: GPG error:  https://repo.vivaldi.com/stable/deb stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 124F19833EAAB8EE: The repository 'http://repo.vivaldi.com/stable/deb stable Release' 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.N: Skipping acquire of configured file 'non-free/binary-armf/Packages' as respository 'http://repository.spotify.com stable InRelease' doesn't support architecture 'armf'Failed to update APT repositories.  Something is wrong with your APT sources, GPG keys, or Internet connection.  Try again shortly or use https://jellyfin.org/contact to find us for troubleshooting.