Update :

The following packages have unmet dependencies:
unity-webapps-preview : Depends: xul-ext-unity but it is not going to be installed
                     Depends: xul-ext-websites-integration but it is not going to be installed
                     Depends: xul-ext-webaccounts but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

sudo apt-get install unity-webapps-preview