When the flight controller is plugged in, no COM port is available for your Betaflight GUI to connect.
You could find an error reminder in the Windows Device Manager, as shown below.
Most of the time, it is caused by the USB driver setup in your PC.
For all the brushed flight controllers from BETAFPV Hobby, you must install the "STM32 Virtual Com Port Driver". You could find the link to this driver from the Betaflight GUI welcome tab.
Attention: Do NOT use the CP210x Driver. This driver only works for Beebrain FC etc.
When the driver works well with the FC board, you could find the COM in the Windows Device Manager, as shown below.
If you want to reflash the FC board via DUF mode, you will find the DFU driver in the Windows Device Manager.