I launched ByteVitae (https://bytevitae.com/) a couple of years ago, got a decent launch here and in Product Hunt, a bunch of users the first days ~3500 and a steady influx of users over the following months. I didn't know how to convert most of those users to clients, didn't talk to them, lost all interest after the initial launch and moved on after an amazing -26€ in benefits :P

I learned so many things and was such a fruitful ride that for me it is far from a failure. But on the business side, definetly a complete failure!

Even wrote a little post mortem at the time: https://vilva.io/blog/1-year-of-building-reflections.

Lesson learned: Talk to your users. Don't neglect the business/marketing side, specially if you are a techie who loves to code. Talk to your users. It is a marathon run, forget about the overnight millionare launchs, the launch is the "easy" part, growing steady from there is the real challenge. Talk to your users!!