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Once in the operations section, ensure that you have selected the right milestone by clicking the appropriate milestone tab. Immediately below the milestone tab label, you'll see a section that says "Comments". Clicking on "Comments" will reveal a new section called "Comment History", which will show historical comments. You can also click on "+ New Comment" to add a new comment.
Note that the "Add Comment" section to the right of Documents and Upload is for comments specific to a task, not a milestone. For more detailed information on adding comments to a task, go to https://sightenio.freshdesk.com/solution/articles/16000010977-how-do-i-add-comments-to-a-task-click here.