Facebook API {"error":{"message":"(#100) No matching user found","type":"OAuthException" Checkbox Plugin

I am trying to use the feature of checkbox plugin. I manage to make the checkbox rendered. A few days ago, I managed to send some basic text 'Hello World' to the user. But now I'm not able to send anything thing. Whenever I tried to send to the user I get this error:

{"error":{"message":"(#100) No matching user found","type":"OAuthException","code":100,"error_subcode":2018001,"fbtrace_id":"HCZPBkofiz9"}}

I even try to cURL using, but no success, still the same error.

curl -X POST -H "Content-Type: application/json" -d '{
  "recipient": {
      "user_ref":"<UNIQUE_REF_PARAM>"
  }, 
  "message": {
      "text":"hello, world!"
  }
}' "https://graph.facebook.com/v2.6/me/messages?access_token=<PAGE_ACCESS_TOKEN>" 

What I've done are:

1. Uninstalled app from page.
2. Select the page again and generate new PAGE_ACCESS_TOKEN.
3. Subscribe to messages, messaging_postbacks, messaging_optins webhooks to the page.
4. Set the new token in .env.
5. Save up the new user_ref through checkbox plugin.
6. Try cURL by using the new user_ref, no luck.

I am able to send with recipient senderID but not user_ref for checkbox plugin.

Any help, are very much appreciated :)