On that documentation page, look at the left side of the screen. Each of the first three sections after "Documentation Home" in the Contents area start with "Signal Processing Toolbox". That is the toolbox that contains this function. Also look at the bottom of the page, just above the box asking if you found this information useful. In that part of the page, you will see "Introduced in R2018a".
So you will need to be using release R2018a or later and have Signal Processing Toolbox installed and licensed in order to use that function. The ver function will show you both which release you're using and which toolboxes you have installed. You may need to upgrade to a newer release, purchase a license for Signal Processing Toolbox, and/or install Signal Processing Toolbox to use instfreq.
