You have two errors here.
Originally Posted by majordread
1. You are missing a GPG key
2. The dpkg lock file is locked.
You should fix (2) before you fix (1).
Open a terminal and type
Enter your password. It will not be echoed to the screen.
sudo lsof /var/lib/dpkg/lock
This will tell you what has got a lock on dpkg.
Post the results back here. I am interested to see what has a lock on dpkg.
You can then either close that program, restart your PC or send a -TERM to the program with the lock by using kill and the pid.
After that you need to import the key.
EDIT: Added command.
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 2EBC26B60C5A2783
sudo apt-get update