运行 apt install curl 命令时显示此错误，并且 apt-get update 也没有更新每个包，我不知道这个错误
ubuntu@master:~$ sudo apt install curl
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
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
ubuntu@master:~$ sudo apt-get update
Reading package lists... Done