Comment 2 for bug 1578616

I solved the error above by performing the following steps:

1. me@mypc:$ sudo mkdir /var/lib/initramfs-tools
2. me@mypc:$ sudo apt-get install linux-image-4.4.0-21-*

Step 1 will create the initramfs-tools directory
Step 2 will install the 4.4.0-21 kernel and all it requires to run