While de Beauvoir just suggested that Sartre’s thought does not let one “becom[e] the being that he is not,” she explains here that this actually just means that people cannot achieve definite
being, meaning that they cannot become the ideal versions of themselves that they imagine. This does not, however, exclude the possibility that someone will change and improve, a process de Beauvoir calls transcendence. Thus, Sartre’s description of man as “a being who
makes himself a lack of being
in order that there might be being” really means that people are entities who imagine themselves as being one definite thing. When they realize that they are not that thing, they become a lack. However, in realizing that they are a lack, people also set up a goal (being) toward which they can aim.