Student or Learner
What's your beef? (What's your problem)
Is it American or British or both?
I think it's a little outdated now as slang in the US. But I may be the one whose outdated and the slang may be current.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I've never heard it used in BrE.
As we know, normally beef refers to a big animal (bull, ox). Therefore, in my mind the idiom sounds like something big in complexity. And a big problem is one of them.