OK. this MAY be easily solved when you understand the "structure" of these forums
You can only post in a "leaf" or "child" forum (depending your preferred metaphor)
this illustrates how "iPhone" is a "limb" or "parent" and you cannot post in it - but can in the "sub-forums" that appear as links
iPhone >
Using iPhone
iPhone Hardware
iPhone in Business and Education
iPhone Accessories
At the end of the New Discussion posting process, you need to make a choice of where to put it
Look what happens when I "Test"

THEN, look what happens when I type the word "iPhone" = ALL by ITSELF

SOME forums are BOTH parent AND child - like "Using Apple Support Communities" or "Apple ID"