Gatling scala - Unable to replace session variable in the request URI - gatling

Question

I do have a session variable named AuctionId and couldn't replace AuctionId here in the POST request URI. Can you please advise what I am missing here?
object BidSubmission extends HttpUtil {
val orders_feeder = csv("data/Round-1.csv").circular
def orderSubmission: ChainBuilder =
pause(pauseBy(5) seconds)
.repeat(1) {
feed(orders_feeder)
.exec(postToUri("${Constants.orderSubmission_URL}/#{AuctionId}/base-orders/proxy-bid", "")
.queryParam("employeeId", "#{empNo}")
.body(StringBody(session => {
println(session)
println(session.attributes("empNo"))
val empNo = session.attributes("empNo").asInstanceOf[String]
val orderNo = session.attributes("orderNo").asInstanceOf[String]
println(s"\n\n\n $orderNo \n\n\n")
println("\n\n\n ${Constants.bidSubmission_URL}/#{AuctionId}/base-auctions/proxy-
bid \n\n\n")
var slotNos = orderNo.replace("[", "").replace("]", "").split(" +")
println(s"\n\n\n ${generatePayload(empNo, slotNos)} \n\n\n")
generatePayload(empNo, slotNos)
" "
}))
)
}
it prints uri like this instead of replacing AuctionId with the session variable value- https://order.com/golden/base-auction/system-override/auctions/#{AuctionId}/base-auctions/proxy-bid
trait HttpUtil extends StrictLogging {
def postToUri(uri: String, requestName: String, statusCode: Int = 201): HttpRequestBuilder = {
appendHeaders(http(requestName).post(uri)).check(status.is(statusCode))
}
private def appendHeaders(request: HttpRequestBuilder,
authScheme: String = config.auth.plannerToken): HttpRequestBuilder = {
request
.header("Authorization", authScheme)
.header("Content-Type", "application/json")
.header("Accept", "application/prs.hal-forms+json")
}
}