Sharon, I’m copying the answer I sent directly to you, here in the correct forum -proof that it depends where on the site that you are. On your wall, I could like but not comment, and here in the forum I can reply but not like.
I’ve found the best way that works for me, is to click on Community at the top, and choose ‘Site-wide News Feed’. You can also get to this from your Profile page, but the Community link is there pretty much no matter where you are. The News Feed lists most things that are going on, and you can like them by clicking on the heart. On some of the screens you will actually see the option to ‘like’ something. I’m not sure that everyone sees the same thing. It is possible that it differs if you are on a phone, etc. But I’d try the Newsfeed and go from there.
And, if, at first you don’t succeed try another area of the site.