When you restrict user access to integrations in Control Hub, you must include the Webex Expert on Demand integration in your allow list.
From the customer view in https://admin.webex.com, go to Settings, scroll to Integrations and then click Manage.
Find the correct integration for your release of Webex Expert on Demand:
In the Overview window, choose Allow access.
Toggle Access for new users to On, click All users to give this permission to all your users, and then click Save.