Messenger error 100: No matching user found

Code 100 with subcode 2018001 means the recipient ID in your request does not resolve to anyone this Page is allowed to message. It is the most misread error in the Messenger catalogue, because the user usually does exist; the ID is simply not their ID for this Page.

Messenger recipient IDs are Page-scoped IDs (PSIDs). The same person has a different PSID for every Page they talk to, and PSIDs are additionally scoped to the app-Page pair, so several common migrations silently invalidate every stored ID: pointing your code at a different Page, replacing the Meta app that the Page subscription runs through, or importing contact IDs from another system. IDs from Facebook Login are app-scoped IDs (ASIDs), a different namespace entirely, and Meta's docs state plainly that they will not work with the Messenger Platform. Numeric profile IDs scraped from facebook.com URLs will not work either.

The other classic trigger is messaging someone who has never messaged you. A PSID only comes into existence when the person first interacts with your Page through a Messenger entry point; you cannot construct one, and you cannot cold-message. If you need a way for people to start the conversation, point them at an m.me link (see the Messenger link generator) or a chat plugin, and capture the PSID from the resulting webhook event.

What is NOT wrong: your token and permissions (those fail with 190 or 200), the messaging window (code 10), and the person's reachability (blocked users return 551). This is a data problem in your own storage layer, which also makes it the most fixable of the delivery errors: unlike 551 there is a concrete engineering remedy, described in the Messenger troubleshooting guide.

{
  "error": {
    "message": "(#100) No matching user found",
    "type": "OAuthException",
    "code": 100,
    "error_subcode": 2018001,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

Treat the PSID as scoped data, not as a user ID: key it by (app, Page, person) and invalidate the column on any migration. Never mix ID namespaces; keep Facebook Login ASIDs in a separate field if you store them at all. Validate at capture time, PSIDs arrive only via webhooks, so any ID that entered your system another way is suspect. The identifier rules are summarized in the limits reference, and the same scoping trap on Instagram is covered in the Instagram messaging guide.