@Robert375
I am sorry for your situation but, when you host on Airbnb, it's a good idea to first familiarise yourself with its policies regarding cancellations and also how the payments work.
Long term stays above one month are always paid in instalments. That has been the case for years. There are different cancellation policies in place for short stays and stays of a month or more, as @Mike-And-Jane0 point out.
This information is all available on the website. Before you choose the cancellation policies in place, you should read them! Strict for short stays and Strict for long stays are not the same. Go to your listing's settings and click on the edit button by the cancellation policy and you will see the options for both short stays and long term ones.
While it is extremely annoying that there is no guarantee that Airbnb will be able to collect the second payment from a guest, I am afraid this is a risk you take with long term stays. Personally, I think if a guest is booking for a longer period, Airbnb should take the instalments a month in advance, not at the start of the month being rented, so that the host is covered to some extent if the guest defaults on payment but, unfortunately, it's just not like that.