command Z to undo not working\can it be fixed?

Just so we're all on the same page iOS systems are for iPhones and iPads. macOS systems are for Macs.

Since 13.6 is Ventura you're referring to a Mac. So 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.

You don't mention which app has the problem or the type of document. Undo will not always work (or work as expected) or be possible even when the app supports it. It is great when it works, but don't rely on it to be 100%.