Can humans be reduced to a function? Based on an article by UK Essays, we are nothing more than robots that operate based on our past experience and other factors like amount of neurotransmitters, hormones, and other chemicals, trying to get optimal outcomes for ourselves. 

Does this mean that every human mind can be reduced to (a very complex and nonperfect) function?
I think the key concept to tackle this question is to consider the concept of abstraction.

Abstract models are generalized models of some kind "reality" that we are interested in, with the aim to describe some behavior of the system in question reasonably well. Often the abstraction should also be relevant to many instances of the entity that we would like to describe: If you want a abstract model to describe a human, you would probably want it to be useful to describe many humans, and not just a specific human. This might provide less detailed predictive power for individuals, but might be more useful to apply to populations.

When talking about the world or the reality in any kind of descriptive way, we are really always using abstract models to think, rationalize and communicate about it.

Now for your question, of course you can make an abstract model of a human being, but you would have to ask yourself on what level of detail you want. Ignoring the practical difficulties in constructing this, we have to consider that the path to a model that matches "reality" as good as possible is endless. For example, for you to successfully model a brain, you would in the end not only have to describe every atom of the brain, but the quarks and maybe end up all the way in string theory. And in the same time capturing all the emergent behavior down this chain and not only being a reductionist.

Theoretically, I'd say that the answer to your question is definitely yes, it would be theoretically possible to make a abstract model for a human mind that has "good enough" predictive power.

Everything we know about the real world is deterministic (except for some uncertainties if we get down to quantum mechanics, that probably would not have any significant influence on a very high-level system like a brain anyway). There is no scientific support that imaginal non-deterministic things like "free will" either exists or have any practical influence, so there is nothing except the pure complexity of the matter, our knowledge and our scientific processes that sets the limits for how detailed we can describe something.