It is possible to reply to social results, but not to online results.
Online results are blogs, news articles and such. To reply to such articles, you generally need an account on this website.
However, you’re able to reply to social results. It’s important that you’ve activated and chose a channel to do this with when you created the full or quick search as shown in the screenshot below. If replying, liking and/or retweeting isn’t working, this option is probably not activated. To activate this, you’ll have to create a new search with the same settings and search terms.