InApp Purcahse Issue : Why its showing Unresolved Reference: setType for BillingFlowParams?

Using billing library in android and to buy subscription instead of in-app products (consumable-non consumable), added .setType(SkuType.SUBS) to BillingFlowParams as below :

val billingFlowParams = BillingFlowParams
                                    .newBuilder()
                                    .setType(SkuType.SUBS)
                                    .setSkuDetails(skuDetailsList!![i])
                                    .build()

But, .setType(SkuType.SUBS) shows me Unresolved Reference: setType

What might be the issue ? How can I launch Purchase flow for the In-App Subscription?

try BillingClient.SkuType.SUBS
You can launch purchase flow like below

billingClient.launchBillingFlow(this, billingFlowParams).getResponseCode();

if we go more thoroughly,
-BTW It's in java

first, you have to implement PurchasesUpdatedListener, AcknowledgePurchaseResponseListener in your class and override onPurchasesUpdated and onAcknowledgePurchaseResponse methods.
then initialize the billing client

private void initBilling() {
        billingClient = BillingClient.newBuilder(this)
                .enablePendingPurchases()
                .setListener(this)
                .build();
        billingClient.startConnection(new BillingClientStateListener() {

            @Override
            public void onBillingSetupFinished(BillingResult billingResult) {
                if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
                    Purchase.PurchasesResult purchasesResult = billingClient.queryPurchases(BillingClient.SkuType.SUBS);
                    billingClient.queryPurchaseHistoryAsync(BillingClient.SkuType.SUBS, (billingResult1, purchaseHistoryRecordList) -> {
                        Log.i("SKURESLUT", purchaseHistoryRecordList.toString());
                    });
                }
            }

            @Override
            public void onBillingServiceDisconnected() {

            }
        });
    }

then you can pass the desired package id to the following method and start the billingFlow,

private void getBillingPackage(String id) {
        List<String> productIdsList = new ArrayList<>();
        productIdsList.add(id);
        SkuDetailsParams.Builder params = SkuDetailsParams.newBuilder();
        params.setSkusList(productIdsList).setType(BillingClient.SkuType.SUBS);
        billingClient.querySkuDetailsAsync(params.build(),
                (result, skuDetailsList) -> {
                    if (!skuDetailsList.isEmpty()) {
                        mSkuDetails.addAll(skuDetailsList);
                        BillingFlowParams billingFlowParams = BillingFlowParams.newBuilder()
                                .setSkuDetails(skuDetailsList.get(0))
                                .build();
                        int responseCode = billingClient.launchBillingFlow(this, billingFlowParams).getResponseCode();
                    } else {
                        Toast.makeText(this, "Requested package not available", Toast.LENGTH_LONG).show();
                    }
                    Log.e("SKURESLUT", result.toString());
                });
    }

then in onPurchaseUpdated Method will invoke when billing flow is completed or canceled. You can acknowledge the purchase here

@Override
    public void onPurchasesUpdated(BillingResult billingResult, @Nullable List<Purchase> purchases) {
        if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK
                && purchases != null) {
            for (Purchase purchase : purchases) {
                Log.e("Purchase:", purchase.toString());
                if (purchase.getPurchaseState() == Purchase.PurchaseState.PURCHASED) {
                    currentPurchase = purchase;
                    if (!purchase.isAcknowledged()) {
                        AcknowledgePurchaseParams acknowledgePurchaseParams =
                                AcknowledgePurchaseParams.newBuilder()
                                        .setPurchaseToken(purchase.getPurchaseToken())
                                        .build();
                        billingClient.acknowledgePurchase(acknowledgePurchaseParams, this);
                    }
                }
            }
        } else if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.USER_CANCELED) {
            Toast.makeText(this, "Purchase Cancelled", Toast.LENGTH_LONG).show();
        } else {
            // Handle any other error codes.
        }
    }

Then you can get AcknowledgePurchase like below

@Override
    public void onAcknowledgePurchaseResponse(BillingResult billingResult) {
        if (billingResult.getResponseCode() == BillingClient.BillingResponseCode.OK) {
            //Purchase Acknowledged and verified
        } else {
            Toast.makeText(this, "Purchase Failed", Toast.LENGTH_LONG).show();
        }
        Log.e("Acknowledged Purchase:", billingResult.toString());

    }