Problem flashing a dev. board

I have a dev board DOITING ESP-12F. I connect it and see a serial port COM8. However when I run Upload I get an error

serial.serialutil.SerialException: could not open port 'COM8': FileNotFoundError(2, 'The system cannot find the file specified.',

What do I do wrong?

Go to your Control Panel, click on Device Manager, and look for COM ports. If you find one, that is more likely where your ESP-12F is connected

Yes I see a com port (COM8) in Device Manager. But when I try to Upload - I get the error.
I have to buttons on board - RESET and FLASH.
My actions

  1. Press RESET then press FLASH
  2. Release RESET then release FLASH
  3. Press Upload in Arduino.

Make sure Serial Monitor is not open or close any terminal program (Putty, etc) accessing the COM port.