Problem with Gatling 3.3.1

[ghazouani_nada]

Hello
I am running a gatling simulation using the gatling maven plugin 3.0.5 and Gatling version is : 3.3.1

scenario : to give you visibility on the scenario: I am testing user authentication after creating users for my web application.(my application is remote in AWS and my Load Test scripts also running in EC2 instance in AWS)
1/ I recover the email and the password which are in a csv file
**2/**in the response header I recover the token and save it in a csv file (because after each request I must pass the token in the header request)

here you can see the simulation code :

class TestLoginManyUsersWithDiffProfile extends Simulation {

  val sessionHeaders = Map(
    "acceptLanguageHeader" -> "fr-FR,fr;q=0.9,en-US;q=0.8,en;q=0.7",
    "Content-Type" -> "application/json",
    "acceptEncodingHeader"-> "gzip, deflate, br"
  )

  var properties = new java.util.Properties()
  var input = new FileInputStream("src/test/resources/data/gatling.properties")
  properties.load(input)

  val httpProtocol = http
    .baseUrl(properties.getProperty("URL_MicroService_Security"))

  object LoginPage {

  val loginProperties = csv("src/test/resources/data/FileOfLoginPassword.csv").circular

    val login = feed(loginProperties)
      .exec(http("LoginPage")
         .post("/login")
         .body(StringBody("""{ "email": "${email}","password":"${password}" }""")).asJson
         .headers(sessionHeaders)
         .check(header("authorization").saveAs("access_token"))
      )

      .exec { session =>
        println("--------------------------------OUTPUT-------------------------");

        println(session);
        session }

      .exec { session =>
        val file = new File("src/test/resources/data/Token.csv")
        if(file.exists()){
          val writer = new PrintWriter(new FileOutputStream(new File("src/test/resources/data/Token.csv"), true))
          writer.write(session("access_token").as[String].trim)
          writer.write(",")
          writer.write("\n")
          writer.close()
        }
        else {
          val writer = new PrintWriter(new FileOutputStream(new File("src/test/resources/data/Token.csv"), true))
          writer.println("token")
          writer.write(session("access_token").as[String].trim)
          writer.write(",")
          writer.write("\n")
          writer.close()
        }
        session

      }

  }

  val scenario1 = scenario("Authentication Scenario of many users with diff profile ")
    .exec(LoginPage.login)

  setUp(scenario1.inject(
    atOnceUsers(300)
  
  ))
    .protocols(httpProtocol)

}

Whenever i run this simulation I get this error:
header(authorization).find.exists, found nothing===>I know that this error means that the 2 requests that are failed have not returned the response header, that’s why the display of this error. but by debugging i found that email and password recover are right and i can connect to the app with them.I don’t know why then it can’t retrieve the token

and the result of the simulation is:

Also I don’t know why the response time for 50 virtual user is between 5s and 26s–>this response time doesn’t make sense for just 50 users(huge response time)