Curley represents an obstacle to George and Lennie because he abuses the power he possesses on the ranch.
Curley's first appearance with George and Lennie shows how much of an obstacle he will be for them. He confronts and hassles both me. He is immediately threatened with Lennie's size. He berates Lennie for not speaking when he is addressed. Curley then mocks George when he says that he and Lennie "travel together."
Curley is an obstacle because he holds power over both men. Being the boss's son, Curley will never have to worry about being fired or about economic reality. At the same time, Curley is insecure. Candy talks about how Curley is a "small guy." As a result, he is constantly "picking fights" with people who are bigger. He sees it as a way to establish power. Both of these reasons represents how Curley is an obstacle to George and Lennie.
Curley embodies comfort and security with his position on the ranch. George and Lennie are the essence of insecurity. They lack sustainability in their position as migrant workers. They have no one else in the world but one another. The fact that Curley is essentially a bully who seeks to showcase his power over others represents why he is an obstacle for George and Lennie.