The biggest business lesson I learned as an entrepreneur is that there is no such thing as a solopreneur; meaning you cannot do it alone. Everyone needs help. You may be great at designing a better mouse trap, but that does not mean you are great at marketing. And if you are great at marketing, you may not be great with finances. Nobody can do everything a business, even a one-person operation, requires.
What is the biggest business lesson you have learned?