Arduino Editor: Monitor menu show "Serial port unavailable"
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....