I'd try checking the state: Did you change the URI from my test?

Here's an adaptation example using a free POST API Tester site to show it working with some debug output:
The website is: https://reqres.in/ and I'm using the /api/register "test" listed on this site. which shows the response that should be.


Code
alias NJSUser {
  var %URI = https://reqres.in/api/register , %JSON = {"email":"eve.holt@reqres.in","password":"pistol"}

  bset -t &headers 1 Content-Type: application/json
  bset -t &body 1 %JSON

  noop $urlget(%URI,pbi,&NJSUser,processNJS,&headers,&body)
}
alias processNJS {
  var %id = $1 , %BV = $urlget(%id).target

  echo -ai2 * url: $urlget(%id).url
  echo -ai2 * redirect: $urlget(%id).redirect
  echo -ai2 * method: $urlget(%id).method
  echo -ai2 * type: $urlget(%id).type
  echo -ai2 * target: $urlget(%id).target
  echo -ai2 * alias: $urlget(%id).alias
  echo -ai2 * id: $urlget(%id).id
  echo -ai2 * state: $urlget(%id).state
  echo -ai2 * size: $urlget(%id).size
  echo -ai2 * resume: $urlget(%id).resume
  echo -ai2 * rcvd: $urlget(%id).rcvd
  echo -ai2 * time: $urlget(%id).time
  echo -ai2 * reply: $urlget(%id).reply

  if ($bvar(%BV,0)) { echo -ai2 * Data : $bvar(%BV,1-).text }
}

once ran: I get the proper result I'm supposed to:

Code
* url: https://reqres.in/api/register
* redirect:
* method: post
* type: binvar
* target: &NJSUser
* alias: processNJS
* id: 1039
* state: ok
* size: 36
* resume: 0
* rcvd: 36
* time: 796
* reply: 
    HTTP/1.1 200 OK
    Date: Wed, 29 Nov 2023 19:32:29 GMT
    Content-Type: application/json; charset=utf-8
    Content-Length: 36
    Connection: keep-alive
    Report-To: {"group":"heroku-nel","max_age":3600,"endpoints":[{"url":"https://nel.heroku.com/reports?ts=1701286349&sid=c4c9725f-1ab0-44d8-820f-430df2718e11&s=kUDIZ1LCVjdUHeFjOJ6KPivCOTJTpaCrKKHLNdofljM%3D"}]}
  Reporting-Endpoints: heroku-nel=https://nel.heroku.com/reports?ts=1701286349&sid=c4c9725f-1ab0-44d8-820f-430df2718e11&s=kUDIZ1LCVjdUHeFjOJ6KPivCOTJTpaCrKKHLNdofljM%3D
    Nel: {"report_to":"heroku-nel","max_age":3600,"success_fraction":0.005,"failure_fraction":0.05,"response_headers":["Via"]}
    X-Powered-By: Express
    Access-Control-Allow-Origin: *
    Etag: W/"24-4iP0za1geN2he+ohu8F0FhCjLks"
    Via: 1.1 vegur
    CF-Cache-Status: DYNAMIC
    Server: cloudflare
    CF-RAY: 82dd46e0ff122c8c-ORD

* Data : {"id":4,"token":"QpwL5tke4Pnpja7X4"}