Fill container option missing in page wrapper section

I have a mobile screen I’m setting up for prototype designs. The parent frame is fixed. Inside, there is a ‘page wrapper’ are a couple of UI elements to show iPhone UI - they are set to ‘fill container’ and respond accordingly as I resize. There is also a navBar, which is also set to fill container and responds when resized. However, I also have a page wrapper which currently only contains a hero section and after searching and trying all solutions, I cannot for the life of me get it to resize with the parent frame - the ‘fill container’ option is always missing. it seems there are a lot of use cases where this is, at best, confusing.