And she's not even full grown yet. Brahmas seem to be aware of their ponderous size even before they reach full size. There's a big brain in that huge head, too, so it's not a matter of lack of smarts. They can injure their legs, break toe nails off, and incur foot injuries from trying to jump off high places. All that has happened with my Brahmas. They are heavy and lack the athletic grace to counter it.
They like low perches, and hate steep ramps. That ramp up to the tree coop is much too long and steep for a Brahma's taste. They would much prefer to be lifted up. Or ride up in an elevator. They're royalty. didn't-cha know? Just kidding.
You could try adding cleats to the ramp, and I would add side rails, too, enough so a heavy bodied Brahma doesn't feel like she's going to be dragged off the ramp by the force of gravity.
Where a high coop like yours would present little or no problem to most regular breeds, Brahmas are going to resist it. Letting her have an Igloo dog house to sleep in isn't an impractical solution if all else fails.