My message was rejected, but I could reply to this person yesterday

You replied to a customer yesterday and it went through. Today you reply to the same person and the message is rejected. Nothing is broken. You have just met the most important rule on the WhatsApp API.

The rule

Every time a customer messages you, WhatsApp opens a 24-hour window. Inside that window you can reply with anything you like. Once it closes, ordinary messages are rejected by WhatsApp. Not delayed, not queued: rejected.

Each new message from the customer opens a fresh 24-hour window. So while a conversation is genuinely going both ways, you will never run into this limit at all.

Sending outside the window: templates

A template is a message whose wording is registered and approved in advance. Once approved, it can be sent at any time, to anyone, including people who have never contacted you. The usual cases are appointment reminders, invoice notices, and order updates.

You can write and submit templates from the Templates menu in your WACO dashboard. There is no need to open Meta Business Manager. Review usually takes anywhere from a few minutes to a few hours.

Why the rule exists

This is why WhatsApp is almost free of spam: a business cannot send uninvited free-form messages to strangers. The rule that feels restrictive today is the same reason your messages get opened and read instead of ignored like a promotional SMS.