Marketing is a VERY different skill from building, and can take years to master. As someone who has built several side projects that landed in the "didn't make any money" bucket, it's easy to fall for the "build it and they will come" mental trap. I am starting the journey of learning marketing now, but I have a lot of optimism, as I am learning all sorts of things that don't work, and looking forward to trying the things that people say do work.