Register Custom Taxonomy.php

<?php 
function shape_custom_post_taxonomy() {
    register_taxonomy(
      'testimonial_category',  //The name of the taxonomy. Name should be in slug form (must not contain capital letters or spaces).
      'shape-testimonial',                  //post type name
      array(
        'hierarchical'          => true,
        'label'                         => 'Categories',  //Display name
        'query_var'             => true,
        'show_admin_column'             => true,
        'rewrite'                       => array(
          'slug'                  => 'testimonial-category', // This controls the base slug that will display before each term
          'with_front'    => true // Don't display the category base before
        )
      )
    );
  
  add_action( 'init', 'shape_custom_post_taxonomy');