Why do I see “The contact hasn't responded to the opt-in request for more than 24 hours. You can try sending the request again.”?

In order for you to message with a client, they must accept your opt-in request. You’re allowed to send a new one every 24 hours. If accepted, you’ll be able to message with the customer. If it’s rejected, we’ll let you know, and you won’t be able to send any more opt-in requests to the customer.