Hi Rahul,
As you know shares bought are settled(delivered) on T+2 day to your Demat account, as long as there is no short delivery of shares everything will be settled on time. But when the shares are short delivered, your settlement is postponed for 1day i,e. shares will be delivered on T+3, in this case when you do a BTST(Buy Today Sell Tomorrow) there are chances of shares going for auction. Let us take your transaction as an example, you had bought 7 shares of JET AIRWAYS on 6th of December, the same should have been settled on 8th of December (T+2), in your case there was a short delivery of the same and as you have done a BTST there were no shares to debit from your Demat account due to which this email was sent to you. On 11th of December you may either get the shares or cash for the same.
NOTE : Doing a BTST always carries the risk of shares going to auction depending in the shares.