"The employee has to clean the office."
Translation:Pracownik musi posprzątać biuro.
As the English sentence doesn't specify whether the office has to be 'cleaned' succesfully, or allows for an interpretation where it is his duty to do regularly... yes, you're right, it should be accepted.
Does it work to say "Pracownik musi posprzątać w biurze" - cf. "w domu" in another exercise?