It appears the above code is drawn from a defective example code posted by Unity.
I wrote about this before: the Unity example code in the API no longer jumps reliably.
I reported it to Unity via their docs feedback in October 2020. Apparently it is still broken:
Here is a work-around:
I recommend you also go to that same documentation page and ALSO report that the code is broken. [Actually, I just re-reported it again today, May 23, 2022.]
When you report it, you are welcome to link the above workaround. One day the docs might get fixed.
If you would prefer something more full-featured here is a super-basic starter prototype FPS based on Character Controller (BasicFPCC):
That one has run, walk, jump, slide, crouch… it’s crazy-nutty!!