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