Please ensure that Zoho Books and Zoho CRM are integrated.
In Zoho CRM navigate to Settings > Marketplace > Zoho and search for Zoho Finance (if you have Zoho One subscription) or Zoho Books. The options will be to Setup Now or Manage.
For a contact based portal, you need to choose either the 'Contacts' or 'Accounts and their Contacts' synchronization option in the settings for CRM-Books integration.
If you do not use the Accounts module in the CRM, i.e none of your contacts have an account, then associate the Zoho Finance contacts with the CRM Contacts module.
If you have a hybrid setup where a few of your Contacts have Accounts associated with them, then use the 'Accounts & their Contacts' option. Make sure to check the 'Include contacts which are not associated to any account' option.
Back in the portal, navigate to Zoho API Settings > Books
Enable 'Activate Books Integration'
Follow the instructions and click on 'Authorize Zoho Books'
For customizing the Books tab for the front end user, navigate to Customization > Zoho Books and enable the modules you would like to display on the front end.