wordpress error : Call to undefined function in plugin

Solution:1

Use below code, working fine , i have tested

   require_once(ABSPATH . ‘wp-config.php’); 

   require_once(ABSPATH . ‘wp-includes/wp-db.php’); 

   require_once(ABSPATH . ‘wp-admin/includes/taxonomy.php’); 

Solution:2

try including this to your file and let me know then —

require_once(WORDPRESS_HOME. ‘wp-config.php’); 

require_once(WORDPRESS_HOME. ‘wp-includes/wp-db.php’); 

require_once(WORDPRESS_HOME. ‘wp-admin/includes/taxonomy.php’);