Even I used 163 mirrors, it still fails because 19.04 is no longer supported.
Err:3 http://mirrors.163.com/ubuntu disco/main amd64 Packages 404 Not Found [IP: 59.111.0.251 80] Ign:4 http://mirrors.163.com/ubuntu disco/main all Packages Ign:5 http://mirrors.163.com/ubuntu disco/main i386 Packages Ign:6 http://mirrors.163.com/ubuntu disco/main Translation-en Ign:7 http://mirrors.163.com/ubuntu disco/main Translation-en_US Ign:8 http://mirrors.163.com/ubuntu disco/main amd64 DEP-11 Metadata Ign:9 http://mirrors.163.com/ubuntu disco/main all DEP-11 Metadata Ign:10 http://mirrors.163.com/ubuntu disco/main DEP-11 48x48 Icons Ign:11 http://mirrors.163.com/ubuntu disco/main DEP-11 64x64 Icons Ign:12 http://mirrors.163.com/ubuntu disco/main all c-n-f Metadata Ign:13 http://mirrors.163.com/ubuntu disco/main amd64 c-n-f Metadata Reading package lists... Done W: The repository 'http://mirrors.163.com/ubuntu disco 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. E: Failed to fetch http://mirrors.163.com/ubuntu/dists/disco/main/binary-amd64/Packages 404 Not Found [IP: 59.111.0.251 80] E: Some index files failed to download. They have been ignored, or old ones used instead.
I checked files in http://mirrors.163.com/ubuntu, it really shows that some files are not existed.
How to fix it ?
see https://www.unixtutorial.org/fix-apt-update-ubuntu-19-04/
put below lines in /etc/apt/sources.list
deb http://old-releases.ubuntu.com/ubuntu disco main restricted deb http://old-releases.ubuntu.com/ubuntu disco-updates main restricted deb http://old-releases.ubuntu.com/ubuntu disco-security main restricted
