If you have Drive Enterprise edition, upgrade to G Suite Enterprise to get this service.
When you turn classic Hangouts on in your organization, it controls classic Hangouts chat, video, and Hangouts Meet.
Hangouts Chat and Hangouts Meet together are the next generation of Hangouts. Soon they will replace the original version (classic Hangouts) in G Suite.
To manage chat settings for both classic Hangouts and Hangouts Chat, see the Hangouts Chat setup guide.