When I click on it and select Mark for Installation a list of packages to be installed is shown which are the following:
libhardware2
libhybris
libhybris-common1
libmedia1
libqt.5position5
oxideqt-codecs
qml-module-qt-labs-settings
qml-module-ubuntu-onlineaccounts
qtdeclarative5-accounts-plugin
I press Mark then I press Apply. I get an error saying Could not apply changes! Fix broken packages first. Then I go to the list of Broken packages and unity-tweak-tool is there. And when I run Fix Broken Packages I get error saying I have held packages but when I run apt-mark showhold it doesn't show anything.
I've installed unity-tweak-tool before and did some changes. And the changes are still here. As I remember, I pressed reset and the UI broke down (I installed a theme using unity-tweak-tool).
I also tried running sudo apt-get purge --auto-remove unity-tweak-tool but it says that it is not installed.
EDIT: What I mean with the changes are still here is the Horizontal Fold effect on Maximize not sure if that is the default or what but any other changes I did using unity-tweak-tool is gone. The themes and stuff.
EDIT 2: This is my very first time trying out Ubuntu (also my very first Linux experience).
EDIT 3: The feature Minimize on click is also still here which I think can only be enabled from unity-tweak-tool.
EDIT 4: There's also this red exclamation mark beside the package name after I select Mark for Installation