Can't restore iPhone 6S to latest iPhone 5 backup
I'm going to assume your iPhone 5 is running the latest beta, which is iOS 9.1. This is newer version than iOS 9.0.1--the latest non-beta release, and the version your iPhone 6s is running, according to your screenshot.

A backup can only be restored onto a phone running the same, or a newer, version of iOS.

So, if my initial assumption is correct, you'd need to pick one of these options:


Restore from your July 10th backup--it's clear you'd prefer
not to do this.
Downgrade your iPhone 5 to iOS 9.0.1, and then create a new backup which
should then be restorable. (Is this even possible? Perhaps by restoring
your iPhone 5 from it's July 10th backup... which is no better than
above.)
Wait until iOS 9.1 is officially released, and then restore from this
backup after you have upgraded your iPhone 6s to that new version (who
wants to wait that long?).
Upgrade your shiny new iPhone 6s to the current 9.1 beta release and
then restore from the current, missing, backup, which will hopefully
become available as the iOS version numbers will now be compatible.