This might have happened:
1. One of the possibilities is that you are trying to make a premature withdrawal of a Joint Fixed Deposit.
2. While placing a request online, you might have selected the Fixed Deposit Account number which has been linked to some others products, like a credit card or any type of loan.
3. Your Internet Banking may have only viewing rights, because of which you are not able to place a premature withdrawal of the fixed deposit request (this is mostly in case of current accounts).
4. The chosen deposit might be Tax Saving Deposit which has not completed the minium 60 months period.