That’s quite a peculiar issue, as your setup looks to be in order, yet the input remains missing. Since you are working with 5.7, which has just released, it could be a conflict tied to said build.
Just for testing, install 5.6, then create a blank project, with enabled CommonUI / CommonInput, and create an Input Data asset. Then check if the Platform Input section expands in this test project.
If it does, either your existing project has a corrupted component, or 5.7 is not properly reading part of it.