After 24 hours of the user's last message you can only send pre-approved Post session messages(Notification messages).

WhatsApp allows 2 types of messages :

WhatsApp Session Message
- These messages are sent to the users within 24 hours of the user's last message. There are no restrictions on the message type. These can be used directly in any of the conversations. 'Send Message' block provides you an option to create a new message.

Note: Human agents can only send session messages to users.

WhatsApp Post session Message - A WhatsApp post-session message or notification messages can be sent anytime to the user and need to be approval from the WhatsApp team beforehand. These messages also incur an additional cost per message. Any media, location, or contact is not supported. Text messages can contain links, formatting, and emojis. More in notification messages can be found here.