What is the ruling on a manager preventing an employee from reading the Quran during free time while at work?

If you do not have work, then there is no harm in reciting the Quran, saying Subhanallah (Tasbih), La ilaha illallah (Tahlil), and performing Dhikr (remembrance of Allah). This is better than being silent. However, if reading occupies you from something related to your work, then it is not permissible for you to do so, because the time is allocated for work.