What is the important lessons you learnt from apps that did not work?

In my case, the first app I vibe-coded, I learned that it is important for you to research market to see whether demand exists or not. Any app without demand and no low to no budget while starting will not work.

The lessons from second app was that in an extremely competitive market, if you don't have a differentiator, the idea simply will not work. Especially if you are creating a saas which many devs have already made and distributed before also.

What lessons have you learned from your previous saas?