指明架构：[arch=arm64] !!!
tsinghua源
sudo gedit /etc/apt/sources.list

deb [arch=arm64] https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal main restricted universe multiverse

deb [arch=arm64] https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-updates main restricted universe multiverse

deb [arch=arm64] https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-backports main restricted universe multiverse

deb [arch=arm64] https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-security main restricted universe multiverse