Do you have board version 0.3 or o.4? There may be a very small number of the v0.3 boards that have a bad connection in the power switch where it provides power to the FTDI chip, so the Mayfly will not be seen by the computer as having a valid serial port. Does the Mayfly at least blink the red and green LEDs when it’s plugged in to either a USB cable or a lipo battery? Do you have an FTDI cable or FTDI adapter that you can use to program the Mayfly using the FTDI header? That will bypass the FTDI chip on the board and allow you to program it. I programmed each v0.3 Mayfly on Amazon via the FTDI header so it should work. I programmed all of the v0.4 boards via the microUSB jack to verify tha each one was working properly.