For my part, I have only ever heard "in shambles". That's not to say that nobody says, "in a shambles". I can't be sure. In fact, ChatGPT assures me that "in a shambles" is also a correct and equivalent expression, but I have caught ChatGPT speaking nonsense before. So I say this only to offer my personal experience.

in shambles is correct

In my experience, in the UK, we don't usually say either. We say something is a shambles. I wouldn't bat an eyelid at 'in shambles' or 'in a shambles' though.

In shambles

"in a shamble" or "in shambles"