linux lite update error

[Sundowner555]

I tried to update packages and got this error:
===========================
Install Updates Error log
===========================
Install Updates could not fetch the package cache information lists.
Go to https://www.freecinema2022.gq/forums/ and paste the log below into a new or existing thread for assistance.

============ Log ===========

Hit:1 https://repo.steampowered.com/steam stable InRelease
Hit:2 http://ppa.launchpad.net/apandada1/libhandy-1/ubuntu focal InRelease
Get:3 https://download.mono-project.com/repo/debian stable-buster InRelease [5.874 B]
Get:4 http://download.opensuse.org/repositorie...untu_20.04  InRelease [824 B]
Hit:5 http://archive.canonical.com/ubuntu focal InRelease
Hit:6 https://dl.google.com/linux/chrome/deb stable InRelease
Hit:7 http://us.archive.ubuntu.com/ubuntu focal InRelease
Err:3 https://download.mono-project.com/repo/debian stable-buster InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A6A19B38D3D831EF
Hit:8 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:9 http://repo.linuxliteos.com/linuxlite emerald InRelease
Err:4 http://download.opensuse.org/repositorie...untu_20.04  InRelease
  The following signatures were invalid: EXPKEYSIG C5BDBD015C83816E home:erigas OBS Project <home:[email protected]>
Hit:10 http://us.archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:11 http://us.archive.ubuntu.com/ubuntu focal-backports InRelease
Hit:12 https://brinkervii.gitlab.io/grapejuice/...ies/debian universal InRelease
Reading package lists...
W: GPG error: https://download.mono-project.com/repo/debian stable-buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A6A19B38D3D831EF
E: The repository 'https://download.mono-project.com/repo/debian stable-buster InRelease' is not signed.
W: GPG error: http://download.opensuse.org/repositorie...untu_20.04  InRelease: The following signatures were invalid: EXPKEYSIG C5BDBD015C83816E home:erigas OBS Project <home:[email protected]>
E: The repository 'http://download.opensuse.org/repositorie...untu_20.04  InRelease' is not signed.

I saw the pinned message about gimp and I tried to apply the fix, but it did not do anything.

[trinidad]

https://linuxize.com/post/how-to-install...ntu-20-04/

repo: deb https://download.mono-project.com/repo/ubuntu stable-focal main

TC