Pledged securities cannot be sold in the post-market session or the auction market because these sessions entails the risk of short delivery - a situation where the shares you've sold don't get delivered on time to the buyer, which can lead to penalties. 

The good news is you can absolutely sell your pledged holdings during normal market hours (9:15 AM – 3:30 PM). Just make sure to unpledge them first or ensure sufficient free holdings are available to complete the delivery.