1. 问题：E: Package ‘curl’ has no installation candidate

Package curl is not available, but is referred to by another package. 
This may mean that the package is missing, has been obsoleted, or is only available from another source

E: Package 'curl' has no installation candidate

2. 解决

• 备份系统自带的源：

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

• 复制阿里或者清华源：

  阿里源： https://opsx.alibaba.com/mirror
  清华源：https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/
  本人使用的是阿里源，以及是Ubuntu系统，ubuntu 18.04(bionic) 配置如下：

  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
  

• 打开/etc/apt/sources.list文件，将原来的内容删除，然后将上述ubuntu 18.04(bionic)
  的配置，粘贴上，保存：

  sudo vi /etc/apt/sources.list
  

• 输入命令sudo apt-get update进行更新：

  sudo apt-get update
  

• 最后，下载curl：

  sudo apt-get install curl
  

  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  The following additional packages will be installed:
    libcurl4
  The following NEW packages will be installed:
    curl
  The following packages will be upgraded:
    libcurl4
  1 upgraded, 1 newly installed, 0 to remove and 448 not upgraded.
  Need to get 378 kB of archives.
  After this operation, 411 kB of additional disk space will be used.
  Do you want to continue? [Y/n] y
  Get:1 http://mirrors.aliyun.com/ubuntu bionic-security/main amd64 libcurl4 amd64 7.58.0-2ubuntu3.16 [220 kB]
  Get:2 http://mirrors.aliyun.com/ubuntu bionic-security/main amd64 curl amd64 7.58.0-2ubuntu3.16 [159 kB]
  Fetched 378 kB in 0s (1,739 kB/s)
  (Reading database ... 228625 files and directories currently installed.)
  Preparing to unpack .../libcurl4_7.58.0-2ubuntu3.16_amd64.deb ...
  Unpacking libcurl4:amd64 (7.58.0-2ubuntu3.16) over (7.58.0-2ubuntu3.9) ...
  Selecting previously unselected package curl.
  Preparing to unpack .../curl_7.58.0-2ubuntu3.16_amd64.deb ...
  Unpacking curl (7.58.0-2ubuntu3.16) ...
  Setting up libcurl4:amd64 (7.58.0-2ubuntu3.16) ...
  Setting up curl (7.58.0-2ubuntu3.16) ...
  Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
  Processing triggers for libc-bin (2.27-3ubuntu1) ...