Message delivery status is only available in 1:1 conversations.
On desktop:
Next to your most recently sent message, the status Sent, Delivered, or Unsent will display until your contact read it. Sent means that your device has sent the message. Delivered means that your message has been received at your contact´s device. Unsent means that your message could not be sent. Learn more about read receipts.
On mobile:
Tap on a message you've sent. The status Sent, Delivered, or Unsent will display underneath the message. Sent means that your device has sent the message. Delivered means that your message has been received at your contact´s device. Unsent means that your message could not be sent. Learn more about read receipts.