Are there people who have a natural affinity for magick and if so are there people who are born with an aversion to it?
Affinities for magic are usually acquired through experience from previous lives. Spiritual skills and abilities acquired in one incarnation are likely to show up in the next.
Having an affinity for magic isn't always a case of previous involvement, though - it can also be a simple matter of possessing the fundamental faculties magic requires.
A soldier, for example, will usually possess a greater degree of discipline, volition, and focal concentration, and is thus more likely to be successful in magic than the average couch potato who stares at their phone or computer screen all day.
People seem to forget that magical power is merely a symptom of spiritual strength and refinement - a reflection of your mastery over your spiritual development.
But "Spiritual Development" isn't merely some hippie, ambiguous blabbering, but a quite objective measure of your command over the various faculties which comprise your Spirit(Consciousness) and Soul(Astral Body).
But one doesn't necessarily need to be a magician to end up with a sharper degree of spiritual development, it can also be a natural product reaped by the lifestyle and maturity of an individual.
Hobbies, professions, daily routines, etc. can all possibly contribute (or detract) to one's spiritual status, and make one more or less conducive to success in magic.
But as far as having an "aversion" to magic, however... No, that's not really a thing.
There are no real anti-magic genetics, as like I mentioned, magic is merely the flexing of spiritual muscle.
Having a poor affinity for magic is usually just a deficiency in the faculties required or a poorly tempered character. Being "born" with an aversion to magic only really exists in the sense of the mentally handicapped or impaired, but that is simply because their very functioning of the consciousness is impeded on a fundamental level.
But as long as you are a regular individual with no outstanding impairments, you needn't worry about some hidden, uncontrollable factor sabotaging your potential for magic.
What holds the vast majority of practitioners back in magic is poor instruction and their neglect of the necessary self-development.
They jump into magic with half-assed or no genuine development of their mental faculties, as if magic is a layman's skill readily available to the common clay.
Magic is a specialty skill, like being an athlete or professional musician - how far you get is up to you and how seriously you take your development.