Arduino Editor: Monitor menu show "Serial port unavailable" Ubuntu 16.04LTS 64bit, Arduino Pro Mini board (ATmega 328P, 3.3V 8Mhz)
I have problem with WEB editor: I selected Arduino Pro mini and ticked "8Mhz ATmega 328P". I tried to upload "blink" test program from Examples, but Web editor show "Error uploading, check if selected board is currently available." And Monitor menu show "Serial port unavalable".

$ dmesg | grep tty
[    0.000000] console [tty0] enabled


I tried to reboot, not help. The green LED on Arduiono is "on" and red LED is blinking. I tried to verify the built-in COM port, in terminal run command    dmesg | grep FTDI, but no any output returned. 

$ ls -l /dev/ttyUSB*
ls: cannot access '/dev/ttyUSB*': No such file or directory


It looks that FTDI USB to Serial port converter not recognized on Ubuntu?
type "tail -f /var/log/kern.log" into a terminal window. This will show you the messages that the kernel produces. Hit enter a few times so you can easily see when something new is added. 

Now remove (hmm. easier if you do that before typing the command above).  and insert the arduino. Now your kernel should produce a few messages stating it found a new device. Linux has drivers for all popular USB chips so no driver issues expected. 

With a bit of luck you'll see something like: 
   Aug 27 18:53:23 kernel: usbcore: registered new interface driver ch341
   Aug 27 18:53:23 kernel: usb 3-3: ch341-uart converter now attached to ttyUSB0
What comes after "now attached to" is what you need to use for the serial port....