A reservation have been made for 6 days, I strongly suspect this is a fake or fraudulent reservation. While it is still active it blocks the room for genuinely Booking.com reservations
How do you cancel such reservations if C/Card seems OK, but the name of guest, original of country, address and phone details looks dodgy - and guest do not answer messages.
This is not the first time, but in fact the third or more. It seems scammers make reservations either to test the stolen credit-card details, and or for 6 days, offering to pre-pay from stolen credit card, then get refunded when they do not turn up.
We are in New Zealand and all International Tourists are cancelled and currently we only get "national bookings" from people who are already in the country.