What is the number one problem while customer discovery?

I'm sure that the first problem in finding clients is the lack of a business plan. I have an online business and have had a lot of problems with that. I thought it would be the easiest. Finding clients. Show your advantage in the marketplace. But I was wrong. Since I didn't have any orders for over 2 months yet. I had to promote my brand. Buy advertising. I spent a lot of effort, money and time. So that I had my first customers. But first of all I had to learn the right information on how to do customer discovery. There is a lot of information on the Internet, but it is hard to find really useful information.