Usually, the feature is enabled, except when you're still in the trial period of your Zendesk account. In that case, you might need to reach out to Zendesk to get the feature enabled.
From the Zendesk documentation
Access to this endpoint is limited. You may not be able to use it to bulk-import users or organizations. If you're unable, a 403 Forbidden error is returned. If you need access and can't use this endpoint, contact us at support@zendesk.com for assistance.
We've also created a sample message for you to use with Zendesk' Support team.
Hi Zendesk Support Team,
Please enable access to the /api/v2/users/create_many endpoint on our account.
As described in your API documentation.
We need to use the Proactive Bulk Tickets app from Sparkly.
Thanks in advance.