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 Dashboard. 

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

You can click on the code to see the reason for the failure or click here to see all the codes.