Please note, this is not a Coursera Support function.
The chat option can be found in the sidebar.
If there are other learners available to chat with, you will be able to find them via the list available when the chat tab is opened. At the top, you can find “Everyone” which is a public chat thread with everyone in the room. When private or public messages are received, you will be notified if you do not have the chat tab opened.