Promotional Offer Errors

Able to show the promotional offer, but after going through the payment process, inputting my password, and confirming the purchase, I get an error popup that says "Unable to Purchase. Contact the developer for more information."

In the logs, I get the following errors:

• SKErrorDomain Code=12 "(null)"
• ASDServerErrorDomain Code=3903 "Unable to Purchase"

Here are the example parameter values I am passing to the combined string appBundleId + '\u2063' + keyIdentifier + '\u2063' + productIdentifier + '\u2063' + offerIdentifier + '\u2063' + appAccountToken + '\u2063' + nonce + '\u2063' + timestamp

• appBundleId: com.app.myapp
• keyIdentifier: copied the key ID from the App Store Connect
• productIdentifier: product_identifier
• offerIdentifier: offer_identifier
• appAccountToken: tried both a UUID and an empty string
• nonce: a UUID to lowercase
• timestamp: UNIX timestamp

How I'm generating the signature (Java)

/* promotionalOfferKey = value between "-----BEGIN PRIVATE KEY-----" and "-----END PRIVATE KEY-----" from the downloaded p8 file from App Store Connect associated with the keyIdentifier parameter above */

// stringToSign = the combined string above

byte[] encoded = Base64.getDecoder().decode(promotionalOfferKey);
KeyFactory keyFactory = KeyFactory.getInstance("EC");
PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(encoded);

Signature ecdsaSign = Signature.getInstance("SHA256withECDSA");
ecdsaSign.initSign(keyFactory.generatePrivate(keySpec));

ecdsaSign.update(stringToSign.getBytes(StandardCharsets.UTF_8));
byte[] signature = ecdsaSign.sign();

signatureToReturn = Base64.getEncoder().encodeToString(signature);

What am I doing wrong? Thank you in advance.