Question:
What is the meaning of this error? How do I fix this in my chat sequence?
Answer:
There may be a variety of reasons why you see this error message after trying to send a message in your sequence.
Facebook doesn't specify the exact reason, but it can be one of the following:
The user fails to provide consent to the message
User disabled the app
User account has issues
There's a technical issue on Facebook's side
The person blocked messages from your Page (that's why you are not able to contact them via a private message)
The person deactivated/deleted their account
The person's account got restricted or disabled by Facebook
There's a technical issue on Facebook's side
Here are more information from Meta team about this error:
This is one of the permissions error you will get when the person has not opted-in to receive messages or opted-out. In this case, you won't be able to send message to this person.
You can check out our error code details here: https://developers.facebook.com/docs/messenger-platform/reference/send-api/error-codes/
Please note that The 551 Error is the expected result for many privacy issues, such as if the User has not engaged with your page through one of the entry points documented, blocked your page, disable its account, among others. Messenger provides multiple ways for users to opt-out of getting messages. This is something that your Application needs to handle. This error code is referenced here and is not an issue with the platform but rather a feature.
https://developers.facebook.com/docs/messenger-platform/policy/policy-overview/#trigger
I hope this unblocks you. We are always looking for ways to do better and I will really appreciate if you could share your feedback via our survey!
You can learn more about it here: Error #551 (This person isn't available right now) when responding to a page message from bot - Developer Community Forum - Facebook for Developers