Not being prepared
It’s hard to imagine candidates who show up for an interview without understanding what the company does and/or what role they’re interviewing for. But it happens, and when it does, the hiring manager has an immediate cause to eliminate that candidate for consideration.
Not only should you research the company in advance and do your best to understand what’s required in the job opening, but you should also prepare your own answers to standard interview questions and be able to explain how your experience matches what the employer is looking for.
Have questions ready to ask the interviewer as well.