A fair number of manuscripts cross my desk every year, and frankly, most of them have serious flaws. Not necessarily unfixable, but things that will take a good deal of work to correct. The most common problem I see is manuscripts in which the leading character works in a profession the writer knows nothing about. It rings false and readers know it.
Recently, I looked over a manuscript in which the main character was a television reporter. The writer had never been part of local news; in fact, he'd never even visited a newsroom. His next-door-neighbor had some passing acquaintance with the field, and she read the author's manuscript. However, because the neighbor was not an expert either, she missed some glaring errors.
In another case, the main character was an opera singer. The author knew virtually nothing about opera. She used a great many "opera" terms, but they were words I'd certainly never heard. I looked them up and couldn't find them. When I asked the author about them, she confessed that she'd made them up. Can't do that--unless you're writing a fantasy and the opera singer lives in another galaxy. But then you have a whole new series of issues.
Of course, no author starts out as an expert in every field, but if you're not conversant with a profession, either recast your character or become an expert. There are many opera singers, reporters, forensics experts, police officers, ballerinas, dog catchers, doctors, potters, politcal operatives, and teachers who would be happy to share what they know about their professions. Read everything you can, too. Have fun increasing your knowledge. That's one of the joys of writing. You get to learn as you go. What could be better than that?