第一种情况：在确定网络连接和质量没问题的情况下，任何软件都无法安装，出现E:无法定位XXX
解决：换源
具体步骤：

1、备份 /etc/apt/sources.list 文件
执行命令

sudo cp /etc/apt/sources.list /etc/apt/sources.list.old

2、打开文件 sudo vim /etc/apt/sources.list   （gedit也可以）
将原文件里面的内容删掉或注释掉
粘贴选择的源：
阿里云源：

deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse

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

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

清华源：

deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty main universe restricted multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty main universe restricted multiverse

3.sudo apt-get update

第二种情况：可以安装其他的软件，但唯独无法安装ros-<distro>-xxx(个人版本是melodic)类的软件包，换源也不好使，可以尝试以下方法（前提：key和ros源完备的情况下）（本人此类情况）

1.echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/ros-latest.list
2.sudo apt update
然后安装ros-melodic-xxx类的软件包就好使了。