First, find out the id of the process that is holding the lock file:

lsof /var/lib/dpkg/lock-frontend

The above command will give you the PID of the processes using the lock files. Use this PID to kill the process:

sudo kill -9 PID

Now you can remove the lock and reconfigure dpkg:

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