Sartre had a constant struggle with the meaning of existence. He defended that existentialism must be accompanied by limitless freedom, but when we consider that other people also exist and have freedom of their own, we become limited. So, does total freedom require solitude from a species in which its existence is based on social behavior?
The Human Rights manifest states that our freedom ends where the freedom of anyone else starts. Sartre struggled contemplated this idea, adopting a position I would say to be deterministic: if we can't be totally free, then our existence is not fully controllable.
This determinism of Sartre was something he fiercely opposed, but felt he had no other choice.