We've received several Help Desk tickets and emails about users being
unable to comment on some blogs. While we cannot fix this issue from
the user end, here's a quick fix for blog owners to enable comments for
their blog. To enable comments for your blog from any user,
click on Preferences > Registration from the top navigation bar. Then
click the checkbox next to "Anonymous Comments"
to enable anyone to leave a comment on your blog. You may also want to
require an email address for approval. See the screenshot below:

Once this is enabled, comments will automatically be queued for moderation. Blog owners will need to manually approve comments by going to Manage > Comments from the navigation bar. (This prevents spam comments from being automatically approved and published.)