These terminal commands should solve your problem:
First remove the Merge List by opening a terminal (Press Ctrl+Alt+T to launch) and run this command:
sudo rm /var/lib/apt/lists/* -vf
or
sudo rm -vf /var/lib/apt/lists/*
these command lines are interchangable
the options used are -v verbose
-v, --verbose explain what is being done
and -f force
-f, --force ignore nonexistent files and arguments, never prompt
If you are ever unsure about a command line or the options being used always refer to the manual page for the command. These can be accessed on the internet by searching for man and the command for example man rm or you can access the same information by typing exactly the same command in the terminal for example man rm instructions for quitting the page will be at the bottom of the terminal like this
picture of man page options
Next, generate a new one by running a simple update:
sudo apt-get update
Then do,
sudo apt-get install build-essential
build essential
package on synaptic and software-center?