出于某种原因，我的 ubuntu-software-center 消失了（在 Ubuntu 18.04 上）。
我尝试了通常的
sudo apt-get update
和
sudo apt-get install software-center
但我最终遇到了这个错误：
Package software-center 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 'software-center' has no installation candidate
我该如何解决这个问题？我尝试使用突触，但没有成功。