i have experienced similar problems when connecting via the usb to serial adaptor. This is how i get it to work
1. unplug the usb from the pc
2. unplug the serial adaptor.
3. plug in the serial adaptor to your pico device
4. reconnect the usb
the peobelm seems to be that if these are connected in the wrong order then the pc recognises the usb as a human interface device (keyboard mouse etc) and your virtual com port does not exist. this has worked on the 15 ADC 16's we have here for datalogging and i hope this works for you too. It took me a while to figure this one out