I have a handful of cheap ESP32 development boards, and some expensive, that all sometimes gives me the error A fatal error occurred: Could not open COM6, the port doesn't exist. But I have found a workaround.

The simplest way I found is to start with opening the Serial Monitor (in Arduino IDE) to see the status of the com-port easily.

Then unplug the USB cable from the dev-board, hold down the EN / reset switch while plugging it in again. If it worked (which it does 9/10 times) the yellow bar should disappear. If it doesn’t, just repeat again, it has never failed me yet.