Throw exception in saga

I have the below api to create a biller

      fun createBiller(@RequestBody @Valid action: CreateBillerRequest, principal: Principal): Mono<ExternalId> {
        return commandGateway.send(
            externalId = ExternalId(),
            code = action.code,
            description = action.description,
            type = action.type,
            enabled = action.enabled

a simple aggregate

      internal class BillerAggregate() {

        private lateinit var externalId: ExternalId

        fun handle(command: RequestCreateBiller) {
              command.externalId, command.code, command.description, command.type, command.enabled, command.actionBy

        fun on(event: CreateBillerRequested) {
          externalId = event.externalId

In the below saga i invoke an external system to create the biller. In the error case i throw the exception WebClientFunctionalException

      @SagaEventHandler(associationProperty = "externalId")
      fun handle(event: CreateBillerRequested) {
        SagaLifecycle.associateWith("externalId", event.externalId.asReference())
          .onStatus({ it.isError }) {
            val statusCode = it.statusCode()
              .switchIfEmpty(Mono.just(statusCode.value().toString() + " " + statusCode.reasonPhrase))
              .flatMap { response ->
                val message = WebClientErrorHandler.getOriginalApiErrorMessage(it, response).orElse(response)
                return@flatMap Mono.error(WebClientFunctionalException(message))

The problem is when i test the api in the error case, the api returns always status 200.
So my questions are:
Is the invokation of the external system in this case in the right place?
In case of error, how can i make the api returns the exception WebClientFunctionalException?

The main issue here is that the event handler that starts the saga runs asynchronously, so the command is already completed before the event handler is executed. If the call to the external system is critical for the command to succeed, you can execute this in the command handler.