You need to have earned 10 reputation on this site in order to provide an answer to the question.
Although you have 101 rep overall you have not contributed any upvoted content to the site so have not received any reputation here. This is because every Stack Exchange site is different and has different requirements of answers. Just because you may have decent reputation on one site that doesn't mean you know the ins-and-outs of how every site on Stack Exchange works.
Very popular questions, such as the one linked to are often protected because they tend to attract low-quality posts, spam or answers that should really be comments so the post is protected to keep the quality of the answers up.
For this reason we have protected the question. If you can get yourself 10 rep on this site (and that's not really so hard - just an upvoted answer on any other question should do it) then you'll be able to leave an answer on that post. But once you've done so be sure that your answer is not just a variation on those that have already been left. That question attracts a lot of "I prefer option B because it looks nicer" type answers (hence the protection status), which aren't really much help to the OP or any future visitors.