Why my `sudo apt update` got many errors?

It's Ubuntu 19, I want to install Python 3, then I run sudo apt update to update packages.

But I always get lots of errors and nothing updated. What do I need to do now? Thanks,

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Ign:1 http://mirrors.linode.com/ubuntu disco InRelease
Ign:2 http://mirrors.linode.com/ubuntu disco-updates InRelease
Ign:3 http://mirrors.linode.com/ubuntu disco-backports InRelease
Err:4 http://mirrors.linode.com/ubuntu disco Release
404 Not Found [IP: 2400:8901:1::8ba2:1ada 80]
Err:5 http://mirrors.linode.com/ubuntu disco-updates Release
404 Not Found [IP: 2400:8901:1::8ba2:1ada 80]
Err:6 http://mirrors.linode.com/ubuntu disco-backports Release
404 Not Found [IP: 2400:8901:1::8ba2:1ada 80]
Get:7 https://dl.yarnpkg.com/debian stable InRelease [17.1 kB]
Ign:8 http://ppa.launchpad.net/deadsnakes/ppa/ubuntu disco InRelease
Ign:9 http://security.ubuntu.com/ubuntu disco-security InRelease
Err:7 https://dl.yarnpkg.com/debian stable InRelease
The following signatures were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging yarn@dan.cx
Err:10 http://security.ubuntu.com/ubuntu disco-security Release
404 Not Found [IP: 2001:67c:1360:8001::23 80]
Err:11 http://ppa.launchpad.net/deadsnakes/ppa/ubuntu disco Release
404 Not Found [IP: 2001:67c:1560:8008::15 80]
Hit:12 https://deb.nodesource.com/node_12.x disco InRelease
Reading package lists… Done
E: The repository 'http://mirrors.linode.com/ubuntu disco Release' no longer has a Release file.
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.
E: The repository 'http://mirrors.linode.com/ubuntu disco-updates Release' no longer has a Release file.
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.
E: The repository 'http://mirrors.linode.com/ubuntu disco-backports Release' no longer has a Release file.
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.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://dl.yarnpkg.com/debian stable InRelease: The following signatures were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging yarn@dan.cx
E: The repository 'http://security.ubuntu.com/ubuntu disco-security Release' no longer has a Release file.
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.
E: The repository 'http://ppa.launchpad.net/deadsnakes/ppa/ubuntu disco Release' does not have a Release file.
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.