Hello @Antonio331,
You can not make them available. Airbnb has a pretty stringent policy regarding hosts cancelling on guests, and it includes penalties. The penalties include: blocking those dates for new bookings, making you ineligible for superhost for a year, and occasionally a fine as well. This policy is primarily to prevent discrimination but also to promote guest confidence that a booking really is a booking.
There are exenuating circumstances which may be forgiven (flood in the building, death in the family..). Even in these situations, if at all possible, I would recommend calling the company to have them cancel in the future. This way you avoid the problem to begin with. You can try calling now to explain if there is an extenuating circumstance - they have forgiven the penalties before, but you will need a reason.
Not being on top of your calendar will likely not work..
Good luck!