Apple Subscription Signature submission

Hi,

I have been submitting a promotion offer with signature generated from server with a Product id to Apple. But I see SKPaymentTransaction failed with nil error and unable to find the root cause.

Here is the error what I am seeing -  <SKPaymentQueue: 0x2825baa50>: Payment completed with error: Error Domain=ASDServerErrorDomain Code=3903 "Unable to Purchase" UserInfo={NSLocalizedDescription=Unable to Purchase}

Also, everytime I submit the product with the subscription offer an alert is displayed as "Unable to Purchase - Contact the developer for more information. [Environment: Sandbox]".

Is there a better way to identify the root cause.

Thank you!