I have Ghost 20.11.28 on two installations, one an upgrade and the other a new install, but in both cases, while booting up, an error shows "wpa_supplicant.failed to start.wlan0" and, as a result, I don't get any WIFI.

....any suggestions on how to fix this?

Try this tips https://wiki.ghostbsd.org/index.php/How_To#Wifi.2FWLAN

Thanks for your suggestion Slughorn but I have tried most of these things without changing the result.

I can't say that I solved my problem but I reinstalled and immediately did the upgrade......everything is running as intended.