I think one of the most fundamental plot failures I see across a lot of books is that the person chosen to solve a problem is not a person who it makes sense to choose to solve this problem.
This ends up being true for a lot of YA books where the teenager is the chosen solution to a problem because they need to be for plot reasons, but there is no actual logical reason why anyone would be relying on them to solve this problem (or often even letting them get close to this problem). No, this kid can't work for the FBI or the Secret Service. No, this twelve-year-old won't be a more accomplished soldier than a fully-trained servicemember. No, this kid isn't allowed to practice medicine.
And you can work around this by presenting a reason for them to be involved despite being less qualified--usually that they put themselves in a situation, despite the adults around them, but also that there is something else that makes them suited to a job/role despite their other lack of qualifications (we need someone who can blend in, etc.).
But it's also true for a lot of adult books. I'm reading a book where the ER doctor/medical professor First Gentleman is going to lead the response to a plane full of people who potentially have Marburg virus, and even ignoring the fact that he wouldn't be allowed within a thousand feet of that plane by the Secret Service, that also fundamentally just does not make sense with how U.S. pandemic and quarantine response work (or even how medicine works--an ER doctor is generally not an infecious disease specialist). It's waved away for plot reasons--but it also represents a failure of the plot as a whole.
Why is your character the one who is trying to solve this problem? Is it their job? If it's not their job, what does it mean for the story for them to be trying to solve a problem that they aren't qualified for, may not have access to, and/or aren't allowed to deal with?
If your only answer is "it works because it needs to work for plot reasons" then it doesn't work at all.
This is a huge problem in Middle Grade - I just read a lovely book that would not have a plot if any of the adults actually acted like adults - but you've gotta play a wiggle room game when it comes to writing fiction for young people. A 12-year-old kid shouldn't be the one to defeat the dragon, but sometimes kids need books where someone their age is defeating dragons.
That said, "every adult in the room has the plot-stupids" does frequently stop a good book from being great. A YA power fantasy is fine, but if you can dig down and really nail home why this particular main character is the only one who can save the day, you wind up with something way better than a paint-by-the-numbers plot.
One of the great things with YA and MG is that "kids are nosy and want to try to fix things" is often a great way around this. You also can have stories where the adults are the problem, or you can have stories where there is something they need a kid for (because they're small, because you have to be under 18 to access something, because they're The Chosen One, etc.).
Another option that you can consider is that you can write stories where the main character isn't necessarily the only one who can save the day--they're just the only one who is saving the day. When I think about Protector of the Small by Tamora Pierce, one of the main things about Kel is that she is determined to fix things to the point of hurting her, and so other people could (and sometimes do) fix things, but she's often the one fixing things because she's the one who says "i'm going to fix this" and then works until she does.
And sometimes the answer is that the kid is a bandaid. It can be "we have no other options so we're stuck with this kid." It can be "we need five people and the fifth person is a half-trained twelve-year-old so here we go I guess." It can be "our previous option died so I guess we're stuck with their apprentice."
What works the least well is when people who would be a better option (or who have access to a better option) go with the unqualified main character not because of a believable reason (they want them to fail, they're a scapegoat, they're a sacrifice, nepotism) but simply because the plot demands it happens.
