我使用带有 Lubuntu 桌面的 Ubuntu 20.04 LTS。大约 2 个月前，我安装了最小的 Ubuntu 安装，并延迟了安装apt.daily.service以进一步减少启动时间。几天前，当我尝试更新操作系统时，它出现了错误。我以为这可能是暂时的。但几天后，它仍然不起作用。我搜索了错误，但解决方案不起作用。
当我尝试执行时sudo apt update，它可以获取所有软件包，但我认为它们没有安装。由于一些预装的更新管理器，我的所有应用程序都是最新的。但我猜不是 Ubuntu 本身。
错误：-
nachiket@Nachiket:~$ sudo apt update
Hit:1 http://in.archive.ubuntu.com/ubuntu focal InRelease
Get:2 http://in.archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Get:3 http://packages.microsoft.com/repos/code stable InRelease [10.4 kB]
Err:4 http://ppa.launchpad.net/lubuntu-desktop/ppa/ubuntu focal InRelease
403 Forbidden [IP: 91.189.95.85 80]
Get:5 http://in.archive.ubuntu.com/ubuntu focal-backports InRelease [101 kB]
Get:6 https://deb.opera.com/opera-stable stable InRelease [2,590 B]
Get:7 http://dl.google.com/linux/chrome/deb stable InRelease [1,811 B]
Get:8 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Get:9 http://packages.microsoft.com/repos/code stable/main armhf Packages [35.6 kB]
Get:10 http://in.archive.ubuntu.com/ubuntu focal-updates/main i386 Packages [500 kB]
Get:11 http://packages.microsoft.com/repos/code stable/main amd64 Packages [34.8 kB]
Get:12 http://in.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [1,081 kB]
Get:13 https://deb.opera.com/opera-stable stable/non-free amd64 Packages [1,151 B]
Get:14 http://packages.microsoft.com/repos/code stable/main arm64 Packages [35.7 kB]
Get:15 http://dl.google.com/linux/chrome/deb stable/main amd64 Packages [1,100 B]
Get:16 http://in.archive.ubuntu.com/ubuntu focal-updates/main Translation-en [238 kB]
Get:17 http://in.archive.ubuntu.com/ubuntu focal-updates/main amd64 DEP-11 Metadata [282 kB]
Get:18 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages [741 kB]
Get:19 http://in.archive.ubuntu.com/ubuntu focal-updates/main DEP-11 48x48 Icons [60.5 kB]
Get:20 http://in.archive.ubuntu.com/ubuntu focal-updates/main DEP-11 64x64 Icons [95.1 kB]
Get:21 http://in.archive.ubuntu.com/ubuntu focal-updates/main amd64 c-n-f Metadata [13.6 kB]
Get:22 http://in.archive.ubuntu.com/ubuntu focal-updates/restricted amd64 Packages [318 kB]
Get:23 http://security.ubuntu.com/ubuntu focal-security/main i386 Packages [254 kB]
Get:24 http://in.archive.ubuntu.com/ubuntu focal-updates/restricted Translation-en [46.1 kB]
Get:25 http://in.archive.ubuntu.com/ubuntu focal-updates/universe amd64 Packages [831 kB]
Get:26 http://in.archive.ubuntu.com/ubuntu focal-updates/universe i386 Packages [617 kB]
Get:27 http://security.ubuntu.com/ubuntu focal-security/main amd64 DEP-11 Metadata [24.5 kB]
Get:28 http://in.archive.ubuntu.com/ubuntu focal-updates/universe Translation-en [174 kB]
Get:29 http://in.archive.ubuntu.com/ubuntu focal-updates/universe amd64 DEP-11 Metadata [329 kB]
Get:30 http://security.ubuntu.com/ubuntu focal-security/main DEP-11 48x48 Icons [11.0 kB]
Get:31 http://in.archive.ubuntu.com/ubuntu focal-updates/universe DEP-11 64x64 Icons [362 kB]
Get:32 http://in.archive.ubuntu.com/ubuntu focal-updates/universe amd64 c-n-f Metadata [18.0 kB]
Get:33 http://in.archive.ubuntu.com/ubuntu focal-updates/multiverse Translation-en [6,472 B]
Get:34 http://in.archive.ubuntu.com/ubuntu focal-updates/multiverse amd64 DEP-11 Metadata [2,468 B]
Get:35 http://security.ubuntu.com/ubuntu focal-security/main DEP-11 64x64 Icons [16.5 kB]
Get:36 http://in.archive.ubuntu.com/ubuntu focal-backports/main i386 Packages [2,568 B]
Get:37 http://in.archive.ubuntu.com/ubuntu focal-backports/main amd64 Packages [2,568 B]
Get:38 http://in.archive.ubuntu.com/ubuntu focal-backports/main Translation-en [1,120 B]
Get:39 http://in.archive.ubuntu.com/ubuntu focal-backports/main amd64 c-n-f Metadata [400 B]
Get:40 http://in.archive.ubuntu.com/ubuntu focal-backports/universe i386 Packages [4,716 B]
Get:41 http://in.archive.ubuntu.com/ubuntu focal-backports/universe amd64 Packages [5,788 B]
Get:42 http://in.archive.ubuntu.com/ubuntu focal-backports/universe Translation-en [2,060 B]
Get:43 http://in.archive.ubuntu.com/ubuntu focal-backports/universe amd64 DEP-11 Metadata [1,780 B]
Get:44 http://security.ubuntu.com/ubuntu focal-security/main amd64 c-n-f Metadata [7,956 B]
Get:45 http://in.archive.ubuntu.com/ubuntu focal-backports/universe amd64 c-n-f Metadata [288 B]
Get:46 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages [628 kB]
Get:47 http://security.ubuntu.com/ubuntu focal-security/universe i386 Packages [502 kB]
Get:48 http://security.ubuntu.com/ubuntu focal-security/universe Translation-en [95.9 kB]
Get:49 http://security.ubuntu.com/ubuntu focal-security/universe amd64 DEP-11 Metadata [58.0 kB]
Get:50 http://security.ubuntu.com/ubuntu focal-security/universe amd64 c-n-f Metadata [11.6 kB]
Get:51 http://security.ubuntu.com/ubuntu focal-security/multiverse amd64 DEP-11 Metadata [2,464 B]
Reading package lists... Done
E: Failed to fetch http://ppa.launchpad.net/lubuntu-desktop/ppa/ubuntu/dists/focal/InRelease 403 Forbidden [IP: 91.189.95.85 80]
E: The repository 'http://ppa.launchpad.net/lubuntu-desktop/ppa/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.`
我可以通过 ppa-purge 删除 ppa sudo add-apt-repository -r ppa:lubuntu-desktop/ppa，但无法完全删除。删除后，它仍会更新其他软件。但当我将其添加回来时，它会出现错误。