You can't downvote articles, you can't downvote replies to your own comments, you can't downvote comments that are a day or so old and older, etc. So make sure

You can't downvote articles, you can't downvote replies to your own comments, you can't downvote comments that are a day or so old and older, etc. So make sure you're looking at a thing that's actually downvotable (e.g. a toplevel comment in a front page thread).