@Ioannis16 It seems the decision to cancel was due to your place not being ready to rent, right? If that is the case and you cancelled the reservation, then the host penalties will be automatically levied, including blocking the dates of the original reservation. Perhaps if you spoke with an Air BNB rep and the Air BNB messaging system reflects that the guest wanted the cancellation, then the calendar can get unblocked. Worth a try. Here is a guide to contacting Air BNB
Also, I suggest you take your listing off Instant Booking until you have completed whatever work is needed to be done. You can still get inquiries about reserving but you can disclose the issues before the place is booked.