Tested on my kakrc file on a single selection that spans multiple lines. comment-line on the other hand works fine (however only comments the first line as to be expected).
Can somebody reproduce?
My kak version is at 76c58aa .
Not all filetypes have block comment characters set, because the language they wrap don't have either.
Example: kak scripts, in which you can only comment lines with the # character. To comment a block (I should have named the comment-selection command just that, comment-block), use <a-s>:comment-line<ret>.