What is one criticism of ethical relativism?

Since you have placed this question in the Business section, I assume that you are asking with regard to business ethics.  I will therefore use an example from the world of business in this answer.
In general, the major criticism of ethical relativism is that it says that there are no actions that are inherently good or bad.  Ethical relativism holds that an action is only good or bad if the society we live in says that it is.  What this means is that an action that is good, for example, in the United States could be bad in China, or vice versa.  The major criticism of ethical relativism is that it means that there is no such thing as absolute wrong or right.
Let us look at an example from the world of business.  In some times and places, people have thought that it was ethical to sell defective products to consumers.  If the consumers could not tell that the product was defective, that was their problem.  Imagine that you come from a country where this sort of thing is considered unethical.  According to ethical realism, it would be fine for you to go and sell defective products in some other country as long as the people in that country felt such a thing was actually ethical. 
This sort of thing is a reason why people criticize ethical relativism.  They think that some actions are wrong no matter what the people in a given country believe.  They criticize ethical relativism because it denies that some actions are wrong in all places.
https://www.scu.edu/ethics/ethics-resources/ethical-decision-making/ethical-relativism/