ubuntu20.04 apt 安装ubuntu-desktop时报 E: Unable to correct problems, you have held broken packages.

ubuntu20.04 apt 安装ubuntu-desktop时出现如下报错：

~$ sudo apt install ubuntu-desktop
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 unmet dependencies:

网上找了好多资料，都不行，

设置–软件更新不行
sudo apt-get install aptitude 也不行
最后，我的国内源是清华源，换成阿里云的镜像就可以了（原因好像是因为阿里云的版本比较新