5zz9iquytql5 From your pic, it looks like you have a default platform on top of a foundation and the curser is on the foundation. The platform does have a trim though. If what you show is where the cursor was when you tried to change the trim, then it probably wouldn't work. But I know from my experience, that adding/changing platform trims can be somewhat tedious because you have to get the cursor in exactly the right spot and is always harder when the platform is short. So, try this, raise the platform a few clicks then try adding/changing the trim. If the platform is in a more or less completed build, that should still work but may cause a few objects on or adjacent to it to disappear and you'll have to replace what disappears.
Hope this helps.