Response management

I have a service where I send the data to an API that is the one that does the magic.
To this day I have only made the CRUD to a BD in MySqlserve, until there all right.
I send the data to the API but at the time of opting for the answer it is marking me as null, something that is not true since if data comes but I do not know how to do that handling.
Someone to help me since it is a payment method.

my services

  let headers = new Headers(
      'Content-Type' : 'application/json'
    let options = new RequestOptions({ headers: headers });

    //let titularTarjeta = this.titularTarjeta;
    let tarjeta = this.numero.toString();
    let codigo = this.codigo.toString();
    let anio = this.anio.toString();
    let mes = this.mes.toString();
    let monto = this.totalPago.toString();
    let orden_id = 1;
    let data = JSON.stringify({
    let url = URL_PAGOS;                

    return new Promise((resolve) => {, data, {headers: new HttpHeaders().set('Content-Type', 'application/json')})
      .then((response) =>
        console.log('API Respuesta : ', response);
        console.log(response['transaccion'] );
          this.pagoValido = true;
          this.alertCtrl.create({// creo la alerta
            title:"Su compra se realizo con éxito",
            subTitle: "Puede ver mas detalles en historial de pedidos en el perfil",
            buttons: [ "OK" ]
      .catch((error) =>
        this.alertCtrl.create({// creo la alerta
          title:"Error con su pago",
          subTitle: error['mensaje'],
          buttons: [ "OK" ]

This is what I get

API Respuesta :  null


Respuesta del API si pruebo desde el navegador


Thanks in advance