I logged into my Ubuntu virtual machine and tried to upgrade my system using
sudo apt-get update sudo apt-get upgrade
I then received an error message which said
E: Could not get lock /var/lib/dpkg/lock-frontend – open (11: Resource temporarily unavailable)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
To find out which processes were using APT, I used the command
ps -A | grep apt
When a list returned with the processes using APT, I then killed those processes using
sudo kill -9 <processID>
At this point I tried upgrading my system, but I received the same error a second time.
So I found the lock-frontend file in the /var/lib/dpkg directory and deleted it using the following command
sudo rm /var/lib/dpkg/lock-frontend
After killing the processes using APT and removing the lock-frontend file, I was then able to upgrade my system and install new packages.