I am unable to see the delivery report of the failed API. Where can I find them?

Check the reason for the failed API by clicking on the SMS Dashboard. 

By clicking on the failed request, you can find the Request ID, Date, and Time on which the SMS was sent, Sender ID used, mobile number to which the SMS was sent, the message which was sent along with the reason why the SMS failed by clicking on the error box.

You can click here to see all the error codes.