Arduino Nano -- cannot download, no /dev/tty*** device
This is probably a duplicate question and duplicate answer, but ... 

Flip the board over, look at the bottom. If it says "FTDI" then this answer doesn't apply. But many cheap clones have moved to the CH340G chip for USB serial communication, it's a tenth of the cost with a tenth of the (unused) features.

You can find CH340/341 drivers from the manufacturer at WCH.CN or linked at the bottom of this post.

Or, skip the USB altogether and search for "ISP" on Tindie and grab a USBASP or TinyISP programmer.