We can say that management is a science because some aspects of it are technical and objective. Managers must do things like tracking inventories and determining how many workers to have on hand at a given time. These things are measurable and quantifiable. Finding the right levels of inventories or labor is a science.
But management is also like an art because it involves dealing with people. Some parts of management involve trying to motivate workers. It involves trying to interact with them in ways that will make them feel good and have high morale. This increases productivity. Dealing with people is much more of an art than a science.