I do not believe either is easier than the other, they require different skillsets. Having written both fanfiction and original works I believe I can weigh in on the issue far better than most.
Despite that, this thread is long enough that an established author, someone with marvelous prose and a number of people hoping to make this explanation easy have already covered the subject. So I will attempt to be brief.
Writing Fanfiction is not an exercise in creativity to the same extent that writing an original fiction might be. Neither is it particularly helpful should you want to learn how to market your own ideas. It is instead an exercise in research, 'development' and controlling a community. You have to know the source material intimately. I do not mean just the books. I mean the entire lore. There is always someone who writes theories, some obscure comment the author made in an interview or meet and greet. Not to mention all that is available for public perusal on Fandom. All of that must be at least hinted at in a competent Fanfiction. Then there are the systems. You have to extrapolate how you can interpret them given a dataset of a handful of interactions. That is not creative work(at least not to the same degree as making a magic system might be) it is something akin to statistics and probability. For example: if X power interacted with water explosively, would it do so with other liquids? If it does so, would that be a problem in their daily life? Did the author address this in the original work? If no, how could you address it in your own?
Then there is the original community of a work. Often you are writing a fanfiction because you did not like the direction a certain fiction took. Perhaps it was a waste of what you though had potential or perhaps it was simply a decision the author made that forced you to put the book down(Such as triggering content or a trope you despise.). Others will have the same opinion and you can find like minded individuals should you write a Fanfiction. Not everyone will agree however. You are not borrowing the author's entire fanbase, just the fanbase that agrees with you. Then, you are creating a community of like minded individuals from that commonality. It is akin to these threads. We all discuss because we are all authors, fanfictions are simply that discussion given form. It is more akin to an argument than a creative exercise.
Meanwhile writing original fiction is entirely based on your own creativity. No one has discussed the topic before(if you've done the correct research or have original ideas) so you do not have an established conversation. You are starting it. That conversation will be hidden among many others that are being had in any particular 'room'(be that ScribbleHub, RR or otherwise) Unless someone is particularly interested in the topic, it can feel rude to barge into another's conversation. You have no like minds here, you have to create them.
Then there are the technical aspects. Creating a world, magic system and characters requires time. Fanfictions also have this planning but it is more of an exercise in research. What characters already exist and how can they be portrayed? You have been given a perfect example to work with. As an original author, there are no examples bar tropes or anything in the public domain. You are working off a much more vague set of directions. For some(I believe they are called pantsers) this is easier. They prefer the lack of structure because that is how their creative ideas flow. For others however this can be incredibly difficult. If I asked you for your opinion on cheese you would likely be able to give me a straight answer almost immediately. IF I asked you what you would do to change cheese, you would have to think about what you know about cheese. If I were to ask you to make something similar to cheese without the dairy content, that would be a specialised field of expertise that you may not have access to in that moment. That may sound like it is more difficult but it is not. It just requires a different form of experience.
We are used to changing things to fit our perceptions of them and we do so every day. But to create something new, we have to force our brains to think in a way they are often not used to. This is the creation of original fiction. It is an underused skill that not many develop and that is why it seems more difficult. If you knew nothing about cheese you would not be able to tell me how you would change it so you would have to go out and buy cheese. That requires a different set of skills than creating a new type of cheese. I'm not sure if this metaphor has gone too far but I believe you get the point.
Fanfiction writers are akin to social extroverts in the writign world, making their decisions based on how they would affect others and learning about them in turn. Original writers are introverts(Or their equivalent) constantly within their own heads and listening less to the opinions of those around them, focusing on personal development.
If I am on a less creative day, I write fanfiction because my brain may not be creatively fueled that day but I still have my logical faculties. If I am creative, I write original fiction. I suggest you think similarly. Are you more of a structured person(Do you schedule your days? Perhaps you have a routine you follow or you prefer an organised room?) or are you a pantser(Someone who just goes with the flow. You put anything anywhere but always know where it is, your days seem to go the same but you never plan it that way. You're good at thinking on your feet and only plan minimally.) Or are you a bit of both like me. I recommend trying both and seeing which works for you. If you are good at fanfiction but struggling with original stories, I suggest you write more extensive outlines and see if that helps. If you struggle with fanfiction but find original fictions much easier then I recommend you look less into the lore. Just go with what feels right. Even if it's wrong and the story develops differently from how the original author intended, that's how some original fictions get started. Beginning with the intention of writing a fanfic can often create some interesting original novels.
I said I would be brief. I was not. This is unforutunately normal for me. Those of you who do not prefer long-winded text such as this I will write a TLDR:
Fanfiction - Not a creative exercise, focused more on research, controlling communities and interpretation. Better for structured thinkers.
ORiginal works - Very creatively intensive, largely improvised, better for those who think on their feet or pantsers.
REcommendation - Try both. If Fanfics are easier, try writing more extensive outlines. If originals are easier, don't worry about what the author already has in place too much. Take the idea you had and run with it. See where it leads and if it sparks a conversation.