Hello @Rishad1,
Your guest made the confirmed reservation without you having to approve it because instant book is enabled on your listing.
Airbnb sets this on by default so you have to turn it off if you don't want it.
To turn it off go to your calendar and then click on the 4 bar icon in the top left beside the listing photo.
Then select "Booking" and turn off instant book there, should you wish.
Airbnb used to allow hosts with instant book turned on to cancel some bookings penalty free but I read on here that that facility seems to have been removed.
The link below is to an article on cancellation penalties.
https://www.airbnb.ie/help/article/990
You could contact Airbnb and plead your case.
Details on how to contact them can be found on in the "Community Help Guides" post that you'll see if you click on "Get Support" after selecting "Ask the community". that post has many helpful articles including "Contact Airbnb"
Steve.
Steve.