Can't get organization id by api

Can't get organization id by api

Hi! I want to use Zoho Mail APi, but can't fetch organization id through my code(according to this  documentation ). It returns me 'Internal Error', which means in your responses documentation that it is your Server error. I can easily get my account details (function get_accounts)  by API, but organization details(function get_organization) still returns error. Is it the error because I am in trial plan? 

Also I can't connect my own(free) domain through cpanel. It said that 'this operation is restricted. I wrote to support, but got no answer. I really need your help. Please, help me to solve this problem. Your help is much appreciated. Thanks in advance. 
P.S. I attached my code to the topic.