The comments section is for seeing comments left by others when photos are synched via Lightroom CC or, I believe anyway, loaded up on flickr. Someone else can confirm the flickr connection. If the photograph is not synched or shared in that manner then the "Comments" field is "Not Supported"
If you want to add your own notes use the EXIF "User comment" field which is displayed in the metadata panel under any of the EXIF categories.
Hope that helps