问题

原因：

ARM64结构不能使用X86结构的源，清华源不完全支持ARM64。使用下面这个源

sudo vim /etc/apt/sources.list

删掉原来的，改成这个

# ARM64 架构专用源
deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal main restricted universe multiverse
deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-updates main restricted universe multiverse
deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-backports main restricted universe multiverse
deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-security main restricted universe multiverse# 如果需要同时支持 x86_64（例如使用模拟器），可以加入以下内容：
deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb [arch=amd64,i386] http://security.ubuntu.com/ubuntu focal-security main restricted universe multiverse

成功后

sudo apt-get update

感谢解决：E:Failed to fetch https://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/focal/main/binary-arm64-CSDN博客