There is a long debate on whether the physicality of the brain undermines the concept of Free Will, but I will argue that this depends on an incorrect conceptualization of the concept of Free Will.
Let's start by taking apart the word "Free Will" - What does it mean to be "free"? What does it mean to "will" something? We are never truly free – we always work within the constraints of physics (I can't fly), our society (when are we allowed to drink? when are we allowed to drive? when is it alright to curse and when not?), and ourselves (what makes us angry? what makes us happy?). But we want to be able to choose.
As a person, we have desire, needs, and wants (of course, these are physical representations too, but that doesn’t make them less real), and we can achieve those things through our actions. By selecting the right actions, we achieve the things we desire. That's agency, however it comes about.
For me, the problem is that philosophy talks about "free will" as if the question is determinism vs agency, but really, that's not what scares people, and it's not what makes people nihilistic. People are really afraid of being slaves – of wanting one thing and doing another.