Hi Yong,
I am Sumit here to assist you with this question.
Is it a Touch device?
I have seen a couple of reports where item checkboxes cannot be disabled on Touch devices.
File Feedback as well:
https://answers.microsoft.com/en-us/insider/for...
Edit 02/16:
This is by design if the device is in tablet mode. Quoted from a Microsoft employee I talked to:
If the device is in a tablet posture (for example, the keyboard is folded back), then it's expected.