TAN rejection can be due to the following reasons.
a.    Incorrect or incomplete filling of Form 49B
b.    Error is processing the fee payment
c.    Existing TAN in the applicant's name
You can reapply for TAN and make sure to rectify any errors that led to the rejection of your previous application. Ensure you complete Form 49B correctly before submission to NSDL. You may visit the nearest TIN facilitation centre for assistance.
Verify your payment processing – if the payment is via cheque, maintain sufficient funds in your bank account. Avoid the mistakes from your previous form and rectify them.