Sailing is a serious job. Sailing is serious job.
I found that job is a countable noun, so it look like "a serious job" is the right answer, but the other phrase doesn't sound wrong either. Thank you.
The indefinite article "a" is required!
Correct: "Sailing is a serious job."
Sorry, but I don't have the exact grammatical explanation off the top of my head. But as a native speaker, that's the right answer.