Why does this happen?

New users don't have the reputation necessary to add comments to every post, which actually needs to be at least 50. New users can add comments to the posts they wrote, or to the answers given to the questions they asked, but they cannot write a comment for an answer given to a question they didn't ask.

What should I do in these cases?

Click on the "flag" link that appears under each post, and select "not an answer" in the dialog box that appears.

Comments, or new questions written as answers, are deleted from moderators, when they are flagged.