If you've never been published before, you're an 'unknown quantity', so you're going to have difficulty selling your first novel.
('chiefly US') The 'big unknown' [=the important thing that everyone would like to know] is how investors will react.
A 'fear of the unknown' kept her from changing jobs.
(..) It has to end here. I order you not to go. I know now why you cry. But it's something I can never do. Goodbye. The unknown future rolls toward us. I face it for the first time with a sense of hope. Because if a machine, a terminator, can learn the value of human life, (..)1991 Terminator 2: Judgment Day