Why can't I give someone Commenter permissions on a report?

I have several current user reports on a dashboard and I want the users to be able to add comments and attachments to the rows they see. However, when I set the permissions drop down to Commenter, the "Share Report" button at the bottom is grayed out.

All of the users are Commenters on the underlying sheet because that was required in order for Dashboard Viewer to work, but my only option for sharing the report itself is Viewer or Editor?? I don't want them to be able to edit the rows, just add comments and attachments.

What am I missing?

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