Passing a body into Grpc payload

[trinp]

I have done simple GRPC plugin and gatling successfully , no issue happening here, when I put this into my project, there are some problems due to my lack of knowledge here:
As @georgeLeung that have a proto message as:

message GreetRequest {
    string username = 1;
    string name = 2;
}

I saw that he had created a payload as this:

  val greetPayload: Expression[GreetRequest] = GreetRequest(name = "World").updateExpr(
    _.username :~ $("username")
  )

So I have tried to follow his example, by this message and Authorization that I have:
message required to be sent:

{
   "device": {
           "id": "random"        
   }
}

Messages defined in proto file:

message StartPairingRequest {
  Device device = 1 [(google.api.field_behavior) = REQUIRED]; // Device object containing information
}

Authorization:
BearerToken: “the key”

I have tried to implement the body only as this:

val greetPayload: Expression[StartPairingRequest] = StartPairingRequest(root.scala.None).updateExpr(
    _.device :~ $("device")
  )

Then it just returned that Java.lang.assertion: Assertion Failed,
But the main question I have that how do I correctly implement the payload with messages and authorization so that I can make the call right ?
I am confused for the message in proto and message required in payload.

Below is my sample code:

import scala.concurrent.duration._
import com.github.phisgr.gatling.grpc.Predef._
import com.github.phisgr.gatling.pb._

import io.gatling.core.Predef._
import io.gatling.http.Predef._
import io.grpc.Status
import io.gatling.core.session.{Expression, Session}

import spray.json._
import DefaultJsonProtocol._ 
import sttp.client4.quick._

class BasicSimu extends Simulation { 

  val grpcConf = grpc(managedChannelBuilder(name = "localhost", port = 8080).usePlaintext())

  val messageBody = s"""{"device": {"id": "random"}}"""

  val greetPayload: Expression[StartPairingRequest] = StartPairingRequest(root.scala.None).updateExpr(
    _.device :~ $("device")
  )

  val scn = scenario("BasicSimulation")
        .exec(
            grpc("test")
              .rpc(RelayServerGrpc.METHOD_START_PAIRING)
              .payload(greetPayload)
              .check(statusCode is Status.Code.ABORTED)
            )

  setUp( 
    scn.inject(atOnceUsers(1))
  ).protocols(grpcConf)
}