WHY? Am I allowed to answer some questions and not others?

There's a glitch here which I'm going to have to explain before I can get to your question properly. Number 1: the glitch I'm talking about here is one where current questions posted under the "Answer" tab are disappearing after about 3-4 hours leaving week-old questions on the list. Also, the "Discover" (trending) tab has not had any new questions either. They're also a week or more old. Or if you just scroll down long enough in any category on this website, then you'll eventually get to the week-old questions. Sadly, this is not the first, and probably won't be the last time that this glitch shows its ugly face.

Number 2: That leads me to my next point, where I can now answer your question properly. There's something going on here where questions that have been posted for a week or more are unable to be answered. The answer box does not appear under the last answer on the page. So it's likely that you're seeing these week-old questions and are attempting to answer them but they're locked off. The only other reasons I can think of for why you may not be able to answer a question is that the user has you blocked (you won't see an answer box at the bottom of the page) or your account has been suspended (obviously not the case as you're here posting this question). Hope this made some sense and that it helped.

Most likely the user has blocked you.

There are also some glitches in the system.