Aunt Alexandra objects to Scout's going to the home of Calpurnia because it is not in accordance with proper behavior.
One night after supper as Atticus reads his newspaper, Scout asks him about a topic which Calpurnia suggested she ask her father. Then, after relating the occasion of her question, the Sunday visit to the First Purchase African M.E. Church that she and Jem made because Atticus was in Montgomery, Atticus seems amused, but Aunt Alexandra puts down the embroidery that she has been working on and stares.
"You all were coming back from Calpurnia's church that Sunday?"
“Yessum, and she promised me I could come out to her house some afternoon. Atticus. I’ll go next Sunday if it’s all right, can I? Cal said she’d come get me if you were off in the car.”
"You may not."
Alexandra forbids her because, unlike her brother, Alexandra holds with traditional behavior. (White people did not go into the homes of the African-Americans or associate with them on a personal level in the segregated South.)
Aunt Alexandra perceives such behavior as improper and unbecoming of a young lady, whose father holds a fairly high social position in Maycomb. After this conversation, she tells her brother,
"You've got to do something with her [Scout]....You've let things go too long, Atticus."
Aunt Alexandra even suggests that Calpurnia be let go because they do not need her any longer since she is living there.