What are the main differences between Functional Programming and Reactive Programming?

It is difficult to answer this because it is like asking the differences between oranges and beets. Yeah, they're both eating, it's over.

Functional programming is something that is based on the use of functions to express code, which limit the change of state, ie it is a paradigm itself. And, contrary to what many people think, it is not just to use function, it has a series of criteria. And much less functionality is what uses function and procedural is what uses procedures.

reactive programming is a style of code organization, not a paradigm (although some classify it as if it were), so they can be used together, it's like using the orange to give a flavor in the beet. Reactive programming has even more to do with architecture, how data is manipulated by the application, and how it reacts to it. The language or code can even help to do this in a simpler way.

Clearly and objectively it is better to learn what each one is in isolation because they are only relational by chance, and do not antagonize each other. The links are above.