ubuntu系统执行sudo apt update命令的时候，遇到如下问题：
忽略:82 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports/universe arm64 Packages
错误:81 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports/main arm64 Packages
404 Not Found [IP: 101.6.15.130 443]
忽略:82 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports/universe arm64 Packages
命中:83 https://repo.download.nvidia.cn/jetson/common r35.4 InRelease
命中:84 https://repo.download.nvidia.cn/jetson/t234 r35.4 InRelease
命中:85 http://packages.ros.org/ros/ubuntu focal InRelease
已下载 4,685 B，耗时 11秒 (438 B/s)
正在读取软件包列表... 完成
E: 无法下载 https://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/bionic/main/binary-arm64/Packages 404 Not Found [IP: 101.6.15.130 443]
E: 无法下载 https://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/bionic-updates/main/binary-arm64/Packages 404 Not Found [IP: 101.6.15.130 443]
E: 无法下载 https://mirrors.tuna.tsinghua.edu.cn/ubuntu/dists/bionic-backports/main/binary-arm64/Packages 404 Not Found [IP: 101.6.15.130 443]
输入这个命令可以解决：
wget -O /etc/apt/sources.list https://repo.huaweicloud.com/repository/conf/Ubuntu-Ports-bionic.list
运行之后执行sudo apt update
运行效果如下：
问题解决！