Indigo - Ticket refund

Pnr number PMJHJA , ticket cancelled by air lines due to covid, but amount kept credit shell. But I want the amount to refunded.But air lines say since it is booked before **th March amount can not refunded.Even though ticket booked date is **/**/****, travel date is */*/**** which is under lockdown period.Hence, as I don't want utilised credit shell option, I need the amount to be refunded.But Indigo air lines not agreeing for this.Initially at the time of cancellation, airlines agreed to refund but afterwards, they sent mail mentioning that the amount is kept in credit shell.why credit shell to be made compelsory. it is only option and option to be given to consumers,( traveler). Hence I am raising this issue and seeking the help in getting the amount back .