Anyone CAN sing, if they put the time and effort into it. A voice is an instrument just as much as the harmonica is. I think some people put regular time into developing their harp chops, but not into their voices, and then complain that they can't sing.
Also helps to pick songs and keys that match your voice. Don't try to be Howlin' Wolf if your voice isn't suited for it.
I always sing naturally and use auto-tune for my harmonica playing. ---------- Marc Graci YouTube Channel