Fail to quickstart

[Joel]

You’re still using the wrong sources. Ubuntu 20.0.4 is FocalFossa (your version) but you have Debian (a completely different OS) and Ubuntu Bionic which is 18.04
Maybe there are other sources.list files being used in /etc/apt/*

Use these as your sources

@Joel

###### Ubuntu Main Repos
deb http://cn.archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse 
deb-src http://cn.archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse 

###### Ubuntu Update Repos
deb http://cn.archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse 
deb http://cn.archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse 
deb http://cn.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse 
deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse 
deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse 
deb-src http://cn.archive.ubuntu.com/ubuntu/ focal-backports main restricted universe multiverse