换源

直接输入以下即可

echo "deb https://mirror.tuna.tsinghua.edu.cn/ubuntu-ports/ focal main restricted universe multiverse
deb https://mirror.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-updates main restricted universe multiverse
deb https://mirror.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-backports main restricted universe multiverse
deb https://mirror.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-security main restricted universe multiverse
deb https://mirror.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-proposed main restricted universe multiverse">/etc/apt/sources.list

更新

sudo apt-get update