missed tactical-shot

each player had 10min for the whole game...this is the possition after 20.Nd2?:

Sorry for my bad english,

here the link to the actual game, after 20.Nd2?, i played a6? and missed this tactical-shot...but i still managed to win, after the oppening i was worse, than i managed to come back and outplay him, at the end he blundered and resigned....: