Roosting is an instinctive behavior for the vast majority of chicks. My chicks hatched in an incubator and shipped from a hatchery instinctively roost when they are ready with no adults around and no teaching. With most of mine that is around 10 to 12 weeks of age but I've had some start as young as 5 weeks and some wait a lot longer but typically around 10 to 12 weeks. By roosting I mean sleeping on them overnight. They will often play by perching on the roosts during the day. At the hatchery, those eggs were probably laid by hens that never roosted in their life yet their chick instinctively roost.
I've had broody hens take their chicks to the roost at two weeks of age. The chicks flew up there when the broody told them to. Yes they flew at 2 weeks but I have chicks like Australorp, Sussex, and Delaware, not Silkies, Frizzles, and other breeds that cannot fly. I've also had broody hens that wean their chicks before they take them to the roost. These are example of why I say that you do not get guarantees with living animals. Anything can happen.
Is it possible that they don't know how to roost or that veggie scraps are food?
My guess is that they were raised without an opportunity to roost so they have been trained to not roost. I'd go out at dark after it is too dark for them to see much and move them to the roost. Be consistent, do that every night and they should soon get the message.
Your coop is 4 feet wide so that roost is plenty long enough. How far is it off of the back wall. I give mine a minimum of 12" so they have plenty of room to face either way, toward the wall or away from it. It looks like it is about two feet off of the coop floor. It is higher than the nests so it is high enough. Mine could pretty much jump that high with minimal wing flapping if they wanted to. Your ladder will not hurt anything so leave it. The reason to train them is so they want to get up there. If they want to, they will get up there.
On the food scraps, they have never learned to eat them. That is not unusual when raised in a coop and never being able to forage. My suggestion is to continue to offer them (but do not leave them out there overnight. They could attract predators and especially rodents.)
I'll tell a story to help illustrate that. One time I gathered a yogurt cup of corn ear worms when preparing to can corn from my garden. I dumped those worms in a pile near about fifteen 10-week-old chicks. Step by step, inch by inch, the chicks slowly approached that pile. A worm wiggled! Run away! Run away! But not very far. Step by step, inch by inch, the chicks slowly approached that pile again. A worm wiggled! Run away! Run away! This repeated 4 or 5 times before one brave cockerel got close enough to grab a worm. That pile was gone within 30 seconds, including some playing keep-away and chasing.
All it will take is one of your pullets trying a bite, then they will all enjoy it. But it may take a while before one takes that first bite. Be patient and they will get there.