How to fix stuck/'disabled' zooming on trackpad?

Since you've already played with your System Preferences, I'd suggest testing to see if this happens while booted into Safe Mode, and also without any external devices connected, including both USB and Bluetooth.

Boot into Safe Mode

Follow these steps to boot your Macbook into Safe Mode:

1. Fully shut down your Macbook
2. Restart your Macbook
3. Immediately press the Shift key and keep it down
4. Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
5. Take a note of what happens (i.e. does zoom work on the built-in trackpad when using Safari, Photoshop, etc)
6. Exit Safe Mode by restarting your Macbook as normal
7. Again, test to see what happens with zoom etc.

Now, if after you've gone through the above steps you find that:

• zooming works during Safe Mode but not when you're logged back in normally, let me know.
• zoom still isn't working during Safe Mode, then it's time to test your hardware with Apple Diagnostics (see below).

NOTE: The steps below are for Mac models introduced from June 2013 onwards. If your Mac was introduced before June 2013, it uses Apple Hardware Test instead.

Run Apple Diagnostics

Follow the steps below to run Apple Diagnostics:

1. Fully shut down your Mac
2. Restart your Mac
3. Immediately press the D key and keep it down until you see the Diagnostics screen appear
4. Wait for Diagnostics to finish (this typically only takes a few minutes)
5. Once complete, one of two things will appear on the screen:
   • a No issues found message
   • a brief description of any errors found plus further instructions
6. If the diagnostics test does find errors, take a note of what they are

Note: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete.

Regardless, take a note of what happens and let me know how you went.

All you need to do is go to system preferences > trackpad > scroll and zoom > uncheck Zoom in or out and then check it again.

Do you have Magnet installed?

That caused the same issue for me.