For the sweet love of all things holy, stay the bejeezus away from ERP and anything that looks like ERP. You will spend at least a year just building a prototype that you’re going to call an MVP. Then when it comes time to try to sell it you won’t get anybody who could actually use the software interested in actually buying it from you. And if in fact pigs start to fly and you manage to land a client, you’ll make practically nothing while being expected to be available all the gosh darned time.
If you’re not burned out building the prototype and stop, you’ll be absolutely hating life the second the new client high wears off. They’ll treat you like the IT help desk they’re not paying you a salary for and you really won’t want to say no. You’ll have no time for sales for that second client.
If you want to provide ERP services to a SMB, sell somebody else’s solution and provide support. That’s a service model that businesses can understand and more importantly, that you can limit the scope of.
If you want to build and sell custom software, you can either build software products or software services. Software service is called SaaS and is a well-understood business model by now. Software products offer more upside potential at greater risk.
ERP is in that weird zone of software where it looks like a product, but you have to sell it like a service. This requires understanding the enterprise procurement process and a sharp staff to provide service. If you’re going to compete with the big boys in that space you better bring your AAA game.
There was a time in my twenties when I thought ERP was a good thing for a wannabe solo entrepreneur to offer. Even then I dared not make my own. OpenERP, now called Odoo, is a thing. Then I realized how badly solo entrepreneurs get destroyed by SMBs and gave up on solo software consultancy.
You need to get a job. Having just learned how to program, you don’t have any of the other thousand or so soft skills needed to make it as a consultant. I don’t know what country you’re in, so I don’t know how feasible that is for you. If it’s not feasible for you, the best business model for you as a solo entrepreneur is probably making Android apps.
Another viable option is making website builder themes, like WordPress or SquareSpace. Making money in this space requires good design skills as well as programming chops, and is a true product category rather than a service.
You can also try your hand at SaaS, but be very careful who you pick as your target market. Programmers are hard to keep happy, but it’s an evergreen market. You may try to build something for, say, teachers, anybody who needs more organization than they have in their lives.
Did you learn programming so that you wouldn’t have to learn other skills? Then you need a job. Otherwise you need to learn how to sell and not just code.