@Edward654 Hosts have no control over that. Not all bookings for all guests are offered that option by Airbnb.
We aren't privy to why that is, but I don't think it's offered if the check-in date is close to the booking date. Which makes sense- if a guest can't afford to pay for the whole booking that's only 5 days away, they likely aren't guests you'd want anyway if they live so much on the edge.
And I think the second payment is always charged about a week before check-in.
And maybe Airbnb doesn't offer it if a guest has had a payment issue in the past, or has a low guest rating, but that's just a guess.
So just tell the guest that hosts have no ability to facilitate that- that Airbnb is in charge of the payment end. Hosts can't even see if a guest has only paid half up front. That info is not shared with hosts.