Xamarin in App Purchases c#

Hello Guys,

i've got some questions about IAP.
(Plugin.InAppBilling)
I've followed a few Tuts about in App Purchases.

I configured the settings on the appstore connect.
and have this code:

Code Block async void JahresAbo()        {
            var connected = await CrossInAppBilling.Current.ConnectAsync();
            if (!connected)
                return;
            var purchase = await CrossInAppBilling.Current.PurchaseAsync("at.XXXXX.XXXX.jahresabo", ItemType.Subscription);
            if (purchase != null & purchase.State == PurchaseState.Purchased)
            {
                await DisplayAlert("Kauf gelungen", "Sie verfügen nun über 365 Tage über die Zeitkarten-App", "Zum Dashboard");
                await Navigation.PushAsync(new Zeiterfassung());
            }
            else
            {
                await DisplayAlert("Kauf fehlgeschlagen", "Probieren Sie es in ein paar Minuten erneut", "Schließen");
            }
            await CrossInAppBilling.Current.DisconnectAsync();
        }

Code Block async void JahresAbo()
{
var connected = await CrossInAppBilling.Current.ConnectAsync();
if (!connected)
return;

var purchase = await CrossInAppBilling.Current.PurchaseAsync("at.XXXXX.XXXX.jahresabo", ItemType.Subscription);
if (purchase != null & purchase.State == PurchaseState.Purchased)
{
await DisplayAlert("Kauf gelungen", "Sie verfügen nun über 365 Tage über die Zeitkarten-App", "Zum Dashboard");
await Navigation.PushAsync(new Zeiterfassung());
}
else
{
await DisplayAlert("Kauf fehlgeschlagen", "Probieren Sie es in ein paar Minuten erneut", "Schließen");
}

await CrossInAppBilling.Current.DisconnectAsync();
}

(My IAP ID is: at.XXXXX.XXXX.jahresabo)

The Error is: Cannot connect to iTunes.
This error pops up after the iTunes-Login-In pops. (When i log in with a real apple-ip: "Cannot Connect.." and with a sandboxuser: "User passwort or ..... uncorrect.
Its clear, because my App is not submitted at AppStoreConnect. How can i test a IAP via Xamarin without submitting it at StoreConnect?

Tested it with a real device.

On the Tuts they say: Test it via TestFlight. But if i invite my device to testflight, i am not receiving a invite-mail or testflight pops up with a push.

Can anyone help me with this problem? i am thankful for every answer i hopefully get.

Have a nice day & sorry for my bad English-> my english teacher was probably not the best(or i was to ****** ;) )