https://www.sleeper.scot/timetable/london-fort-william/
"Day service tickets are only available to buy a maximum of 7 days before travel and require a reservation. Anyone who does not hold a reservation may not be permitted to travel."
I can definitely buy flexible tickets on the ScotRail website and app for over a week in advance though (including for the Sleeper from Edinburgh, so boarding there is definitely possible).
Sorry I misunderstood slightly & thought you meant they had to booked a minimum of 7 days in advance.
I think technically it's a reservation that can't be made more than 7 days in advance. You can buy a "normal" walk up type ticket that would be valid on the service, further ahead than that, but would need to wait until closer to the time to get the seat reservation.
Am guessing this is to give them scope for service alterations.
It's come up before on this thread that CS on occasion is somewhat obstructive in letting people book onto these "day" seats, or gives out inaccurate information. Whether that's deliberate or just the result of indifference is up for debate. At the same time...while in theory you need that reservation, in practice it seems very unlikely that you'd not be let on the train, if you have a ticket but not the reservation.
Sorry if that is a confusing answer. But I would say that you're unlikely to run into a problem of not being able to reserve a seat, assuming the train is running on the night you want to travel.