E: 无法定位软件包 ros-noetic-desktop-full

安装ubuntu时，到sudo apt install ros-noetic-desktop-full一直卡住，后来发现是换源时修改sources.list文件里边旧链接没有删除干净。

sudo gedit /etc/apt/sources.list

查看修改sources文件，保存

 sudo apt-get update

从新更新一下，问题就解决了。
我的sources文件里边的链接如下

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

deb http://cn.archive.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
deb-src 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-src http://cn.archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse

# deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed 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-backports main restricted universe multiverse