Did you upgrade your OS before making sure the applications you use actually work with the new OS?
The System Requirements mention MacOS 10.9 as the latest one for Photoshop CS6.
When one decides to use obsolete software changing any component of a running set-up (OS, hardware, …) is a risk. (edited)
You're on a Mac so you need to press Command+Z, not Control+Z. Ctrl+Z is the Windows shortcut for Undo. Cmd+Z is the Mac shortcut for Undo. If that's just a typo (easy to happen if you're a Win user too), see next...
To rule out the obvious, try a different keyboard to make sure your keys aren't worn or glitchy. For instance the bluetooth keyboard that came with my Mac had keys that wouldn't always commit the key stroke and I had to return it; and I have a USB Mac keyboard that I've hit Cmd+S so often the S key doesn't always work.
Also if you're on an older Mac and working with the Mixer brush on large files, make sure your paint stroke is completely finished drawing before pressing the keystroke. Sometimes the brush lags the cursor, depending on a number of factors.
To be honest, it never really occurred to me that my Photoshop wouldn't be compatible with the new update.
I had been avoiding upgrading, but eventually too many of my apps on my Mac gave me error messages, saying that the app cannot be used because my Mac was out of date, so I finally did the upgrade but am now deeply regretting it.
Yes, I meant Command+Z.
No, it's not my keyboard.
It's hard to explain the problem, really. it's the normal Brush tool. it can do a couple strokes then gets stuck. but clicking outside of the Photoshop window and then back in resets the tool. By getting stuck, I mean that I can't do the Command+Z command or change tools, for example to the Eraser. It even got stuck on the eraser tool. I'm only doing small strokes in a small file so it's not a lag issue.
I run CS5.1, CS6, CC 2015.5, and CC 2017 on a 2+ year old iMac with OS Sierra 10.12.3, and haven't seen that problem. Perhaps post your System Info here to see if it reveals any clues. Go Help>System Info and copy/paste.