She is petrified.
Shortly after she sees Chigurh in this scene, she says "I need to sit down," and she takes a seat. Her need to sit was most likely caused by a biological reaction to fear:
One of the ways in which the body prepares for this action is to redirect blood flow to the areas most needed, including the legs, which are needed to run away from danger. Increased blood flow to the legs can make them feel weak, tingly or like jelly. Source
Even though the blood flow is to facilitate running, it can make a person feel like they cannot run.
She believes she can reason with him.
During the scene, Carla mentions that she just buried her mother, seeking some pity. She says the money is gone, hoping to make him lose interest. And she tells him that he has no reason to hurt her, which logically is true. Unfortunately, she did not count on Chigurh keeping his word to her husband that he would kill her if her husband did not give himself up.
She doubts she'd be able to outrun him.
She is in a skirt and in heels. He is a powerfully built man in boots. Her legs feel weak from fear, and she probably assumes he has a gun, even if she cannot see it. Running probably seems like an unlikely prospect for her.