Maybe the manager chose the wrong words when explaining it to you.  Generally speaking, if the zoning for the apartment is residential renting an apartment for use as a business may be against the zoning ordinances and may be against some other local statutes.  It also may violate the lease agreements for the other apartments in that the other tenants had an expectation that there would not be those other than their residential neighbors going in and out of the complex.  If yo want to press the issue I would ask the manager to explain exactly what he meant by the statement.  Maybe he is referring to a law that is specific to your area. Good luck.