@Ana21064 This is not necessarily some scam request, but could simply be a case of a guest new to Airbnb not knowing how it works. Certainly important to be aware of the red flags, but not necessarily nefarious.
A simple message back "Hi XX, I see you are new to Airbnb, so you may not be aware of some of the policies. Hosts are not in the habit of allowing pre-viewing of a property, it poses a security risk. Guests make a decision on whether to book a listing based on the listing information, the photo gallery, and asking questions if they require clarification. Also, it is not acceptable, in an Airbnb, to have friends or anyone else over, who has not been listed on the original reservation, without the express permission of the host, and is usually not allowed at all. Doing this can lead to eviction from the property. And communication between hosts and guests needs to be kept to the Airbnb messaging feature. Writing out your phone number to try to get around the rules can lead to your account being closed. If you are willing to abide by these Airbnb protocols, we can discuss a booking."
You may be unaware, as a new host, that an Inquiry and a Booking Request are handled differently, so pay attention to which it is. For an Inquiry, all you need to do is reply within 24 hours, you don't have to pre-approve or decline. For a Request, you do need to decide whether to accept or decline within 24 hours.
As a new host, I wouldn't accept a 20 day booking from anyone. I'd suggest that you adjust your settings to something like a week-10 days maximum stay until you get more experienced with hosting- it could save you a lot of headaches.