Возникли проблемы с установкой curl (и других пакетов) на lubuntu [дубликат]

Итак, у меня lubuntu 20.10 (в котором я новичок), когда я пытаюсь установить пакет, например curl с

sudo apt install curl

, я получаю сообщение об ошибке

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package curl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'curl' has no installation candidate

, и такая же ошибка возникает, когда я пытаюсь установить другой пакеты.

В некоторых старых сообщениях форума предлагается выполнить

sudo apt-get update

, но при этом возвращается ошибка

Get:1 http://fi.archive.ubuntu.com/ubuntu xenial InRelease [247 kB]
Hit:2 http://dl.google.com/linux/chrome/deb stable InRelease                                                                         
Hit:3 http://ppa.launchpad.net/pinta-maintainers/pinta-stable/ubuntu groovy InRelease                                                
Ign:4 http://ppa.launchpad.net/webupd8team/java/ubuntu groovy InRelease                                                              
Hit:5 https://download.mono-project.com/repo/debian stable-buster InRelease                                                          
Err:6 http://ppa.launchpad.net/webupd8team/java/ubuntu groovy Release
  404  Not Found [IP: 2001:67c:1560:8008::19 80]
Err:1 http://fi.archive.ubuntu.com/ubuntu xenial InRelease   
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/webupd8team/java/ubuntu groovy 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.
W: GPG error: http://fi.archive.ubuntu.com/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://fi.archive.ubuntu.com/ubuntu xenial 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.

Кто-нибудь знает, как это исправить?