Facebook API: (#100) No matching user found - facebook

Question

I spent whole days to try to figure out this error, I also notice there are similar questions with the same message, but I'm totally not sure I find the answer in there, so if someone could point out or give any tip into my problem, I totally appreciate that.
I'm making the request to send a message from a Facebook page A to a user X (by userId) who makes a comment on a post of FB Page A, through a testing FB App name MyApp. I am trying this on Facebook Graph Explorer.
I got the user Id by fetch the list of comments in a post in the Facebook Page, but I totally could not send the message.
Here is the error
{
"error": {
"message": "(#100) No matching user found",
"type": "OAuthException",
"code": 100,
"error_subcode": 2018001,
"fbtrace_id": "H3yMO0RWaDy"
}
}
Here is the my track
MyApp has been setup webhook OK, because it is not published yet for the testing purpose (unapproved app), so I have to add the user X account into tester pool of MyApp (the message that I try to send is also this user X)
I logged into user X to make a comment on a post in the FB Page A
On Graph Explorer, I selected Application: MyApp, and get Page Access Token of FB Page A, I performed two following requests
3.1 /{page_id}_{post_id}?fields=comments
With this, I can get the list of existing comments in FB Page A, here is the output:
{
"comments": {
"data": [
{
"created_time": "...",
"from": {
"name": "User X",
"id": "123456789" // this is the user Id that I would take to send the message to
},
"message": "Sample comment from User X",
"id": "..." // {page_id}_{post_id}
}
],
"paging": {
....
}
},
"id": "..." // {page_id}_{post_id
}
}
3.2 {page_id}/messages: I am trying to send a message from FB Page A to user X with the user Id that I got from 3.1 step
recipient: {id: "123456789"} // there seems be a problem with this id that I got from step 3.1, FB cannot find the user id with this number
message: {text: "hello"}
I'm also acknowledged about the app/page scoped id for user.
I have already subscribed MyApp to the FB Page A with API (/{page_id}/subscribed_apps)
Facebook API v2.10
I have no clue what's wrong.

Answer 1

I figured it out. Facebook Messenger Platform use paged scope user id, but what I were using is app scope id. They are different.
Here is official doc from Facebook about how to get the paged scope id
The id must be an ID that was retrieved through the Messenger entry
points or through the Messenger webhooks (e.g., a person may discover
your business in Messenger and start a conversation from there.
These IDs are page-scoped IDs (PSID). This means that the IDs are
unique for a given page.
If you have an existing Facebook Login integration, user IDs are
app-scoped and will not work with the Messenger platform.
The solution is subscribing MyApp to the FB Page (what I've already done) to get the webhook to work, where I will get the page scope id from the message of user.
Furthermore, detail of how to setup the subscription found here
http://ukimiawz.github.io/facebook/2015/08/12/webhook-facebook-subscriptions/