Unable to get property 'TaxonomySession' of undefined or null reference

SP.Taxonomy is not defined
Unable to get property 'TaxonomySession' of undefined or null reference

Root Cause: You are missing any one of this js files.  SP.Runtime.js, SP.js, SP.Taxonomy.js

Solution:

Make sure you loaded  SP.Runtime.js, SP.js, SP.Taxonomy.js files, before your execute below code.

You can load js files using below code...

Unable to get property 'TaxonomySession' of undefined or null reference

You are missing some of the below js files.
SP.Taxonomy.js
SP.Runtime.js
SP.js