Promotional offer error while testing with sandbox account

Hello everyone I'm testing promotional offers on for my app. I've setup a product with a promotional offer for it. I've also have our backend developer setup signature generating. However when I tried to purchase the offer from my app, I'm getting the error:

`Error Domain=SKErrorDomain Code=0 "An unknown error occurred" UserInfo={NSLocalizedDescription=An unknown error occurred, NSUnderlyingError=0x149a7c330 {Error Domain=ASDErrorDomain Code=500 "(null)" UserInfo={NSUnderlyingError=0x149a7cba0 {Error Domain=AMSErrorDomain Code=305 "(null)"}}}} `

Also during the purchase process, I noticed the Xcode console printed out this:

<SKPaymentQueue: 0x10d3fc210>: Payment completed with error: Error Domain=ASDErrorDomain Code=500 "(null)" UserInfo={client-environment-type=Sandbox, storefront-country-code=VNM, NSUnderlyingError=0x149a7d320 {Error Domain=AMSErrorDomain Code=305 "Purchase Failed Server canceled the purchase" UserInfo={AMSURL=https://sandbox.itunes.apple.com/WebObjects/MZBuy.woa/wa/inAppBuy?guid=00008110-0001549C36BA401E, AMSDescription=Purchase Failed, NSDebugDescription=Purchase Failed Server canceled the purchase, AMSServerCorrelationKey=3Y44VA33BIC5CZCH5I7WLTQ5DA, AMSStatusCode=200, AMSServerPayload={
    "cancel-purchase-batch" = 1;
    customerMessage = "Unable to process your request.";
    dialog =     {
        defaultButton = ok;
        explanation = "Please try again later.\n\n[Environment: Sandbox]";
        initialCheckboxValue = 1;
        "m-allowed" = 0;
        message = "Unable to process your request.";
        okButtonString = OK;
    };
    failureType = "";
    "m-allowed" = 0;
    metrics =     {
        actionUrl = "sandbox.itunes.apple.com/WebObjects/MZBuy.woa/wa/inAppBuy";
        asnState = 0;
        dialogId = "MZCommerce.SystemError";
        eventType = dialog;
        message = "Unable to process your re";
        mtEventTime = "2026-07-15 06:14:16 Etc/GMT";
        mtTopic = "xp_ase_payments/appstore_payments_ue";
        options =         (
            OK
        );
    };
    pings =     (
    );
}, AMSFailureReason=Server canceled the purchase}}}

Could someone please help me figure out what the problem is? Thanks in advance!.