我最近在我的计算机上安装了 Ubuntu 16.04，但是当尝试执行 sudo apt-get update 时我收到以下消息：
W: The repository 'http://ppa.launchpad.net/freyja-dev/unity-tweak-tool-daily/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: Failed to fetch https://deb.opera.com/opera-stable/dists/stable/InRelease Connection timed out after 120000 milliseconds
E: Failed to fetch http://ppa.launchpad.net/freyja-dev/unity-tweak-tool-daily/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
W: Some index files failed to download. They have been ignored, or old ones used instead.
经过一番研究，我认为更改我的下载镜像会很有用，但是在选择推荐的镜像并尝试更新后，我收到错误消息：
无法下载存储库信息。请检查您的互联网连接。
详细信息如下：
W:The repository 'http://ppa.launchpad.net/freyja-dev/unity-tweak-tool-daily/ubuntu xenial Release' does not have a Release file.
W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details.,
W:Failed to fetch https://deb.opera.com/opera-stable/dists/stable/InRelease Connection timed out after 120000 milliseconds
E:Failed to fetch http://ppa.launchpad.net/freyja-dev/unity-tweak-tool-daily/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
W:Some index files failed to download. They have been ignored, or old ones used instead.
你知道该如何解决这个问题吗？提前致谢。
答案1
问题在于错误消息中提到的 PPA，而不是官方存储库镜像：
https://deb.opera.com/opera-stable/dists/stable/InRelease
http://ppa.launchpad.net/freyja-dev/unity-tweak-tool-daily/ubuntu/dists/xenial/main/binary-amd64/Packages
禁用或完全删除它们。更改镜像不会产生任何影响。