Can't get Mac to forget a wifi network

If all else fails give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

If you’re really running macOS 13.1, update and try again. macOS 13.6.9 is current, and has fixed a few things.

Exactly how are you trying to forget the network? Are you going to Settings>WiFi>Known Networks and clicking on the three little dots at the right then selecting "Forget this Network?"

If you have multiple devices with the same network in known networks, I'd forget it from all of them and restart everything.