I have had similar problems getting the MKR FOX 1200 serial port to appear
I usually
- open the Control Panel > System > Device manager > Ports so I can watch the COM ports appear/disappear
- double cllick the reset button - after a few tries a COM port appears
- upload the Blink example code so I have a operational program
sometimes when compiling/uploading I have to double click the reset again
I have also found that the COM port can change after upload