"The staff has the book."
Translation:Tá an leabhar ag an bhfoireann.
"Tá an leabhar acu" would be "They have the book," which isn't a literal enough translation for Duolingo. If you meant "Tá an leabhar acu an bhfoireann", that wouldn't be grammatically correct. "acu" means "at them" and is a complete prepositional phrase, so the "an bhfoireann" isn't serving any grammatical function.