Ubuntu 16.04.1 usbserial missing

I had exactly the same problem you mention with my SiLabs CP2102-based ESP32-DevKitC board from Espressif. You don't mention the environment you're using, but I was running Ubuntu as a VirtualBox VM, which turned out to be the source of my problem.

The problem is that USB Serial drivers aren't installed by default in Ubuntu images designed for VM and cloud use.

To resolve the issue, install the linux-image-extra-virtual package by running:

sudo apt-get install linux-image-extra-virtual

Once I installed that package, I got a /dev/ttyUSB0 serial device shortly after connecting my USB device to my (host) Mac.

I had the same problem. You just need to install linux-modules-extra which contains the driver for your kernel.

With the following command:

sudo apt install linux-modules-extra-$(uname -r)