Your board will only present a serial port if the sketch that is running on it is programmed to present a serial port. The board does not have an actual serial port for communication with the PC - only what the sketch provides.
If the sketch doesn't provide one, or if the sketch isn't running (for example if it was compiled for the wrong board by accident) then you will never see a serial port.
Since the "normal" way of resetting the board to enter the bootloader (which does present its own serial port) is to open the serial port at 1200 baud, since you don't have one to open it's kind of hard to get to the bootloader. However, depending on what bootloader you have installed there are a number of options that can help you:
- Briefly connecting RST to GND just before uploading may activate the bootloader for a few seconds.
- Doing the above twice in rapid succession may activate the bootloader for longer (depending on the bootloader version)
Erasing the chip and reinstalling the bootloader is the only real way that is guaranteed to work, though. But that said, I have seen boards come out of China without any bootloader at all - so reinstalling the bootloader is always a good first option with cheap boards. You can use your Arduino UNO as an ISP programmer (using the ArduinoISP sketch and treating it like you're making a breadboard Arduino).