Brake assembly

Hello, I need to fit a brake on my bmx but I have never done it

When trying to assemble it, I have the impression that the small part which "blocks" the end of the sheath under the tube is missing

I have the small cable passage under the frame but when I tried to mount it, the cable + sheath are "floating", both move when I brake….

They told me that by tensioning everything correctly this should no longer happen.

“Cross” shoe brake assembly

Does anyone have any advice please?

Thank you ✌🏻🤙🏻

THANKS