Hello @Jessica1237
Airbnb has a Help Centre which is a great resource and will provide you with information around the basics of how Airbnb works including payments. Do take the time to read through this to help you understand how Airbnb works.
To answer your question, no guests cannot pay you 50% on arrival. They either pay it all up front on booking or if they are booking some time in advance (I think a month or more in some areas) they might be able to 50% up front and the balance a month in advance.
If this option is available it will show the guest this when they book.
I don't agree with @Paul1255 on this.
There is no reason to refer your guests to Airbnb about this. Just confirm to them that Airbnb works on the basis that either they pay in full on booking, or in some case if they are booking a long time in advance there maybe an option to pay 50% on booking and the balance a month before.
This is either a first time user to Airbnb or it is an experienced guest trying to take advantage of a new host.