Here are some key terms regarding Jean-Paul Sartre's concept of freedom: existence precedes essence, being for itself (one's own self), being for others (others' existence), and being in itself (all the unconscious things), anguish, bad faith (fleeing from oneself or responsibility).
He considers that there are many things which we can't choose. We have to accept them, such as, the environment, our neighbors, etc, like 'man is being thrown into the world'.
But despite all these things he claims the importance of responsibility. When someone chooses something, they are not only choosing for themselves but they are also setting a path for others. So I think only this might affect one's freedom, because it is hard to take responsibility without knowing the consequences, due to which either fear or anxiety influences our thinking.
We want to escape from our responsibility of taking action, but we can't claim that our freedom is bound to follow the essence which has been set forth. We can choose freely. Here we can say we need to follow soft-determinism and amor fati (love of fate). So NO metaphysical entity, or other external thing exists to stop us from thinking and taking action. We just have to be soft determinists and lovers of our fate.