Hi @hfpong ,
It may be a hardware problem.
Perhaps the cable from the touchpad/ buttons to the motherboard is loose or perhaps the buttons contact surface has worn down with use.
Here’s a link to the [https://topics-cdn.dell.com/pdf/inspiron-15-7566-laptop_service-manual_en-us.pdf|service manual] for your laptop (taken from this [https://www.dell.com/support/home/au/en/audhs1/product-support/product/inspiron-15-7566-laptop/manuals|webpage])
Scroll to p.66 to view the necessary pre-requisite steps and then the procedure to remove / replace the touchpad.
This should give you access to the touchpad connector cable to check that it is connected securely.
If it is OK it may be that you’ll have to replace the touchpad button assembly.