Hi there,
The space between the bottom of the cabinet and the bottom of the wall bracket is 16 1/2 inches. For reference, check page 7 (MOUNTING SPACE) of the installation manual and watch the full video of the Samsung over-the-range microwave installation.
Installation manual:
https://downloadcenter.samsung.com/content/EM/202403/20240318130943669/DE68-04108A-08_IM_ME18H704SFB_AA_EN_240305.pdf
Installing your over-the-range microwave | Samsung US:
https://www.youtube.com/watch?v=eueYEAgeDWQ&t=13s
Have a great day,
-Ernest-