To troubleshoot OneNote not recognizing a stylus on a touchscreen device on Mac, follow these steps:

1. Check that the device is correctly connected and the stylus is working
2. Open the Input Sources list and check that the language is set to US English
3. Check the system preferences to make sure that the pointing device and other input options are properly configured
4. Update the firmware on the device if prompted
5. If the above troubleshooting steps didn’t work, try resetting the SMC and NVRAM in the system