TL;DR：必须降级libc6然后就可以安装了build-essential
经历过同样的事情：
$ sudo apt install build-essential
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:
build-essential : Depends: libc6-dev but it is not going to be installed or
libc-dev
Depends: g++ (>= 4:9.2) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
这是我修复它的方法。
Checked apt-cache policy g++，我想这已经很满足了，所以我继续：
$ apt-cache policy g++
g++:
Installed: (none)
Candidate: 4:9.3.0-1ubuntu2
Version table:
4:9.3.0-1ubuntu2 500
500 http://mirrors.xtom.com/ubuntu focal/main amd64 Packages
Checked apt-cache policy libc6-dev:
$ apt-cache policy libc6-dev
libc6-dev:
Installed: (none)
Candidate: 2.31-0ubuntu9.2
Version table:
2.31-0ubuntu9.2 500
500 http://mirrors.xtom.com/ubuntu focal-updates/main amd64 Packages
2.31-0ubuntu9 500
500 http://mirrors.xtom.com/ubuntu focal/main amd64 Packages
尝试安装特定版本（上面显示的最新版本）：
$ sudo apt install libc6-dev=2.31-0ubuntu9.2
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:
libc6-dev : Depends: libc6 (= 2.31-0ubuntu9.2) but 2.31-0ubuntu9.3 is to be installed
E: Unable to correct problems, you have held broken packages.
做了同样的事情，尝试安装特定的libc6版本：
$ sudo apt install libc6=2.31-0ubuntu9.2
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
glibc-doc
The following packages will be DOWNGRADED:
libc6
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
Need to get 2,715 kB of archives.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n]
...
dpkg: warning: downgrading libc6:amd64 from 2.31-0ubuntu9.3 to 2.31-0ubuntu9.2
...
强调我收到的警告：dpkg: warning: downgrading libc6:amd64 from 2.31-0ubuntu9.3 to 2.31-0ubuntu9.2
现在我可以安装了build-essential:
$ sudo apt install build-essential
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
g++ g++-9 libc-dev-bin libc6-dev libcrypt-dev libstdc++-9-dev linux-libc-dev manpages-dev
Suggested packages:
g++-multilib g++-9-multilib gcc-9-doc glibc-doc libstdc++-9-doc
The following NEW packages will be installed:
build-essential g++ g++-9 libc-dev-bin libc6-dev libcrypt-dev libstdc++-9-dev linux-libc-dev manpages-dev
0 upgraded, 9 newly installed, 0 to remove and 0 not upgraded.
Need to get 16.2 MB of archives.
After this operation, 77.1 MB of additional disk space will be used.
Do you want to continue? [Y/n]
我阅读了多个不同的 stackoverflow 帖子，这些帖子引导我使用apt-cache policy $package我不熟悉。然后我就玩玩而已。
