Custom fields are not available on the Free edition of Zoho CRM. If you're on the Free edition and need to add fields beyond the standard set, you'll need to move to a paid edition first.
The Free edition is aimed at very small teams getting started with Zoho CRM, with basic Lead, Contact, Account, and Deal management, a limited set of workflow rules, and a small storage allowance. It's designed as a genuine starting point rather than a fully customisable CRM, and custom fields are one of the capabilities reserved for paid editions.
If you upgrade and want to start adding custom fields, see our article on how to create custom fields in Zoho CRM for the exact steps, field type options, and things to know before you start.