Hello, I’m Virginia an independent advisor with 20 years of expertise in fixing my own & friends’ PC problems.
Which Windows version are you running - 1809, 1903 or 1909?
I had to do this to get back the option to disable the touchpad:
https://www.intowindows.com/enable-precision-to...
This adds extra settings within settings>devices>touchpad where you can turn off the touchpad when a mouse is connected.
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products).
Please do not download any repair tool which may be offered by this site other than the ones I mention/recommend.
Thoroughly research any product which is mentioned here before you decide to download and install it.