Messenger error 551: This person isn't available right now

Code 551 with subcode 1545041 is Messenger's way of saying the recipient exists, the Page can see them, but the platform will not deliver your message to them. Meta's error-codes table files the bare code 551 under "User Block Error: This person isn't receiving messages from you right now," and the subcoded variant carries the shorter string "This person isn't available right now." Both describe the same wall: something on the person's side of the conversation forbids delivery.

The most common reason is that the person blocked your Page or blocked messages from it. It also fires when the account has been deactivated or deleted, when the person has restricted who can message them, and in some cases when Messenger itself has limited the account. Crucially, Meta does not tell you which of these applies, deliberately: revealing whether a specific user blocked you would leak information about that person's choices. There is no API to check reachability in advance and no appeal path, because nothing is wrong with your app.

What is NOT wrong: your access token (the call authenticated fine), your permissions (the API got as far as the delivery check), the PSID (a wrong ID returns 100/2018001 instead), and the messaging window (window violations return code 10). A bot that treats 551 as a transient failure and retries is the classic mistake; every retry fails identically and inflates your error rate, which is itself a signal Meta watches.

In a support-bot context this error most often surfaces in drip or follow-up flows: the person chatted once, later blocked the Page, and your scheduled follow-up hits the wall. It is also the error you see when messaging users who tightened their privacy settings after Messenger's periodic privacy prompts. The narrative walkthrough of silent-bot causes, including this one, is in the Messenger bot troubleshooting guide.

{
  "error": {
    "message": "(#551) This person isn't available right now.",
    "type": "OAuthException",
    "code": 551,
    "error_subcode": 1545041,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

You cannot prevent a person from blocking your Page, but you can stop the error from mattering. Record every 551 against the contact and gate all outbound automation on that flag. Keep re-engagement lists fresh: PSIDs older than a few months have a meaningfully higher block rate. And reduce the behavior that causes blocks in the first place, unwanted frequency and irrelevant broadcasts; the volume rules worth respecting are tabulated in the Messenger limits reference.