On the app, you go Settings > Live Chess you can change your Allow Chat setting.
On the site (v2) this shows how to disable but you can do the reverse to enable:
https://support.chess.com/customer/en/portal/articles/1444900-how-can-i-play-without-receiving-chat-messages-from-my-opponent-
I have disabled the chat and don't know how to enable it again! Please help!