When I press the reset button then briefly if I go to Tools menu I will see
"Port (/dev/cu.usbmodem14201 (Arduino Nano BLE 33)"
but after a few seconds it will be gone.
What you are seeing is the Nano 33 BLE temporarily going into bootloader mode. With the right timing, you could do an upload during this short window of time. However, the Nano 33 BLE has a nice feature that allows you to make it stay in bootloader mode so you don't need to worry about getting the timing just right:
The tricky thing about the boards with native USB functionality like your board is that the USB code that creates the CDC serial port is running on the same microcontroller as your sketch. This means your sketch code can break the USB code, or stop it from running.
Fortunately, there is an easy way to recover from this situation:
- Press and release the reset button on your board quickly twice. You should now see the LED on the board pulsing, which means the bootloader is running. The double press causes the bootloader to run indefinitely (until the board is reset, powered off, or an upload is done), which means you don't need to get the timing of the reset just right.
- Select the port of your board from the Tools > Port menu. The port number may be different when the bootloader is running so don't assume you already have the correct port selected.
- Start an upload in the Arduino IDE.
The upload should now finish successfully. After this, you should be able to go back to doing normal uploads without needing to press the reset button. If you still need to do the reset trick to do uploads after this, the problem may be caused by your code. You can verify this by uploading a simple sketch like File > Examples > 01.Basics > BareMinimum.