Sorry @Rawand0 you can't unblock those dates. AirBnB hates it when hosts cancel as they consider that it makes them look bad. Blocking off dates is one way they punish hosts for cancelling (see more here). If you contact them (see methods and numbers here) and have extenuating circumstances (listed here) for why you cancelled they might unblock the dates.
If you confirm in first place a guest and after for any reason you cancel the booking, that mean that you have a solid issue to hosting, so is for this that the days are blocked. you can't undo this.
follow up the link that Victoria give to you for understand better. 😉
Just to add another piece of advice, never cancel a reservation for a guest. Always make the guest cancel because if a host cancels, even at a guest's request, the host will will be penalized.