Beginner Question about how to get better at building software

So I deployed my first Web app to Heroku after working on it for 4 months; it worked fine in development and completely went to shit in production. What do I need to learn about to build a better app? Is it my code? should I read the book clean code? Or should I learn more about software design? Or maybe debugging? Is it normal to debug in production? I learn the fundamentals of testing but didn't really know exactly what to test, now I have a better idea after seeing my code go to shit in a different environment. If you can write code a program and make something work but it goes to shit in production how can I improve on so that it works well in production and real ppl could actually use it bc thats the whole point right?