First of all, hello and welcome to iMore. I don't exactly know why you're having this issue so let me ask a couple of questions. Are you using the iMore Forums App or are you using a PC or Mac? Also, are you sure that you are completely signed in?
If you’re seeing sections of the iMore Forums that you cannot access then those sections are either off-limits or there’s a glitch. With that in mind, which particular section are you attempting to access?