Not working for me, as I said in another comment, asking it to set it to 15% will set it to 20%.
that sucks that you're the only one in the world that it doesn't work for. sorry to hear that
Instead of being a dick you could have searched for "Alexa set volume to a value less than 1" on Google. Multiple reddit threads with exactly the same problem.
Have you actually checked that "Set volume to 15%" will set it to 15% in the Alexa app?