Ubuntu 20.04 LTS带持久存储的Live系统无法安装软件问题求助

我正在使用带40GB持久存储的Ubuntu 20.04 LTS Live系统，但现在碰到了个头疼的问题——不管尝试安装什么软件，都会弹出错误提示：Unable to correct problems, you have held broken packages。

就拿安装Keepass2来说吧，我已经按步骤添加了对应的PPA仓库：sudo apt-add-repository ppa:jtaylor/keepass，也执行了更新和升级命令：sudo apt-get update && sudo apt-get upgrade，但运行sudo apt-get install keepass2 -y时还是会触发同样的损坏包错误！之前安装i2p的时候流程几乎完全一样，也碰到了这个问题。更糟的是，连Ubuntu Software应用都打不开了。

安装Keepass2的完整命令输出 #

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:

keepass2 : Depends: mono-runtime (>= 3.0~) but it is not installable

Depends: libmono-corlib4.5-cil (>= 5.18.0.240) but it is not installable

Depends: libmono-system-drawing4.0-cil (>= 5.12.0.309) but it is not installable

Depends: libmono-system-security4.0-cil (>= 5.18.0.240) but it is not installable

Depends: libmono-system-windows-forms4.0-cil (>= 5.16.0.220) but it is not installable

Depends: libmono-system-xml4.0-cil (>= 4.6.1.3) but it is not installable

Depends: libmono-system4.0-cil (>= 5.18.0.240) but it is not installable

Recommends: xsel but it is not installable

E: Unable to correct problems, you have held broken packages.

执行sudo apt-get dist-upgrade的完整输出 #

[sudo] password for i2p:

Reading package lists... Done

Building dependency tree

Reading state information... Done

Calculating upgrade... Done

0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

当前已配置的PPA源 #

/etc/apt/sources.list:deb cdrom:[Ubuntu 20.04.5 LTS _Focal Fossa_ - Release amd64 (20220831)]/ focal main restricted

/etc/apt/sources.list:deb http://archive.ubuntu.com/ubuntu/ focal main restricted

/etc/apt/sources.list:deb http://security.ubuntu.com/ubuntu/ focal-security main restricted

/etc/apt/sources.list:deb http://archive.ubuntu.com/ubuntu/ focal-updates main restricted

/etc/apt/sources.list.d/i2p-maintainers-ubuntu-i2p-focal.list:deb http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu focal main

/etc/apt/sources.list.d/i2p-maintainers-ubuntu-i2p-focal.list:# deb-src http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu focal main

/etc/apt/sources.list.d/i2p-maintainers-ubuntu-i2p-focal.list:# deb-src http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu focal main

/etc/apt/sources.list.d/i2p-maintainers-ubuntu-i2p-focal.list.save:deb http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu focal main

/etc/apt/sources.list.d/i2p-maintainers-ubuntu-i2p-focal.list.save:# deb-src http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu focal main

/etc/apt/sources.list.d/i2p-maintainers-ubuntu-i2p-focal.list.save:# deb-src http://ppa.launchpad.net/i2p-maintainers/i2p/ubuntu focal main

/etc/apt/sources.list.d/jtaylor-ubuntu-keepass-focal.list:deb http://ppa.launchpad.net/jtaylor/keepass/ubuntu focal main

/etc/apt/sources.list.d/jtaylor-ubuntu-keepass-focal.list:# deb-src http://ppa.launchpad.net/jtaylor/keepass/ubuntu focal main

备注：内容来源于stack exchange，提问作者pencilwatch