我正在尝试安装 unity-webapps-preview，所以我之前添加了 PPA：
$ sudo add-apt-repository ppa:webapps/preview
$ sudo apt-get update
但我收到以下错误：
$ sudo apt-get install unity-webapps-preview
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have …