Gfilui

Whenever I try to install anything I get this error (this error is when I try to install firefox)

Setting up virtualbox-ext-pack (5.0.40-0ubuntu1.16.04.1) ...
/var/lib/dpkg/info/virtualbox-ext-pack.postinst: 18: cd: can't cd to /usr/share/virtualbox-ext-pack
dpkg: error processing package virtualbox-ext-pack (--configure):
 subprocess installed post-installation script returned error exit status 2
Errors were encountered while processing:
 virtualbox-ext-pack
E: Sub-process /usr/bin/dpkg returned an error code (1) 

Whenever I try to unistall anything I get this error (this error I get when trying to remove virtualbox)

Removing virtualbox-ext-pack (5.0.40-0ubuntu1.16.04.1) ...
/var/lib/dpkg/info/virtualbox-ext-pack.prerm: 4: /var/lib/dpkg/info/virtualbox-ext-pack.prerm: vboxmanage: not found
dpkg: error processing package virtualbox-ext-pack (--purge):
 subprocess installed pre-removal script returned error exit status 127
Errors were encountered while processing:
 virtualbox-ext-pack
E: Sub-process /usr/bin/dpkg returned an error code (1)

Earlier I had tried to install VMware but my laptop does not support required technology and every attempt to get around it failed!

I tried to remove virtualbox with methods described here and this is the result:

Do you want to continue? [Y/n] y
(Reading database ... 873830 files and directories currently installed.)
Removing virtualbox-ext-pack (5.0.40-0ubuntu1.16.04.1) ...
/var/lib/dpkg/info/virtualbox-ext-pack.prerm: 4: /var/lib/dpkg/info/virtualbox-ext-pack.prerm: vboxmanage: not found
dpkg: error processing package virtualbox-ext-pack (--purge):
 subprocess installed pre-removal script returned error exit status 127
Removing virtualbox-qt (5.0.40-dfsg-0ubuntu1.16.04.1) ...
Purging configuration files for virtualbox-qt (5.0.40-dfsg-0ubuntu1.16.04.1) ...
dpkg: virtualbox: dependency problems, but removing anyway as you requested:
 virtualbox-ext-pack depends on virtualbox (>= 5.0.40-dfsg-0~) | virtualbox-5.0; however:
 Package virtualbox is to be removed.
 Package virtualbox-5.1 which provides virtualbox is not installed.
 Package virtualbox-5.0 is not installed.
 virtualbox-ext-pack depends on virtualbox (<< 5.0.40-dfsg-z) | virtualbox-5.0; however:
 Package virtualbox is to be removed.
 Package virtualbox-5.1 which provides virtualbox is not installed.
 Package virtualbox-5.0 is not installed.
 virtualbox-ext-pack depends on virtualbox (>= 5.0.40-dfsg-0~) | virtualbox-5.0; however:
 Package virtualbox is to be removed.
 Package virtualbox-5.1 which provides virtualbox is not installed.
 Package virtualbox-5.0 is not installed.
 virtualbox-ext-pack depends on virtualbox (<< 5.0.40-dfsg-z) | virtualbox-5.0; however:
 Package virtualbox is to be removed.
 Package virtualbox-5.1 which provides virtualbox is not installed.
 Package virtualbox-5.0 is not i
Removing virtualbox (5.0.40-dfsg-0ubuntu1.16.04.1) ...
Purging configuration files for virtualbox (5.0.40-dfsg-0ubuntu1.16.04.1) ...
dpkg: warning: while removing virtualbox, directory '/usr/lib/virtualbox/ExtensionPacks' not empty so not removed
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for desktop-file-utils (0.22-1ubuntu5.1) ...
Processing triggers for bamfdaemon (0.5.3~bzr0+16.04.20160824-0ubuntu1) ...
Rebuilding /usr/share/applications/bamf-2.index...
Processing triggers for gnome-menus (3.13.3-6ubuntu3.1) ...
Processing triggers for mime-support (3.59ubuntu1) ...
Processing triggers for shared-mime-info (1.5-2ubuntu0.1) ...
Processing triggers for hicolor-icon-theme (0.15-0ubuntu1) ...
Errors were encountered while processing:
 virtualbox-ext-pack
E: Sub-process /usr/bin/dpkg returned an error code (1)

While trying sudo apt-upgrade I got this:

sudo apt-get upgrade
Reading package lists... Done
Building dependency tree 
Reading state information... Done
You might want to run 'apt-get -f install' to correct these.
The following packages have unmet dependencies:
 virtualbox-ext-pack : Depends: virtualbox (>= 5.0.40-dfsg-0~) or
 virtualbox-5.0 but it is not installable
 Depends: virtualbox (< 5.0.40-dfsg-z) or
 virtualbox-5.0 but it is not installable
E: Unmet dependencies. Try using -f.

Let us hope that one or more of the commands by @oldfred and some other experienced users in the following list can help you.

# Oldfred's command list for cleaning and repairing

#houseclean
sudo apt-get autoclean # only removes files that cannot be downloaded anymore (obsolete)
sudo apt-get clean

#refresh
sudo apt-get update #resync package index
sudo apt-get upgrade #newest versions of all packages, update must be run first

#would upgrade you to the latest kernel in the repositories
#dist-upgrade is also able to remove existing packages if required
sudo apt-get dist-upgrade

# fix Broken packages -f 
sudo apt-get -f install
sudo dpkg --configure -a

# Remove lock
# If you are absolutely sure you do not have another upgrade process running.
# Locked dpkg - only if sure you are not running another update.

sudo rm /var/lib/dpkg/lock
sudo dpkg --configure -a 

# added zika's tip for problems with hash sum mismatch

sudo rm /var/lib/apt/lists/*
sudo apt-get update

# added 2F4U's tips for Package Manager & Update Manager problems

Does executing these commands help?

cd /var/lib/apt
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get update

This will rebuild the cache.

If it doesn't help, this forum thread has additional suggestions:

GPG key error when attemting to download updates