Don't build software for salons; start your own salon that uses software you create, and make that your competitive advantage. Don't know how to cut hair? Then you probably don't know what software would help in that business anyway. Find a business you can do yourself, and help yourself using your technical skills.
This is also why this (very good) advice is not so easy to follow in real life. You need multiple skillsets.
The obvious problem here being that there is a very, very fine line to tread where software actually matters.
If you're not as well placed as the incumbents at the core product (salon, sure), software will not save you.
If you're better placed than the incumbents at the core product, software doesn't matter.
If you're (roughly) equally placed as the incumbents at the core product, software can help you, but it will likely take a while to realize any competitive advantage.
And, I say placed, because there are a lot of factors at play. There's the quality of the service/product (itself a huge set of variables), pricing, location, and availability, to name a few. None of those will custom software directly help you with.
This is also why this (very good) advice is not so easy to follow in real life. You need multiple skillsets.