My left back door doesn't want to open. How can I fix it?

Weird fix that worked for me on a 2015 Optima I came up with in a parking lot knowing about this a little bit, so take it with a grain of salt.

What I did to fix the problem on that Optima was I had a few minutes to myself and I got sick of bailing people out in that back seat (I told them flat out why it works now). When I had 5 minutes to mess with it, I flipped the child lock from on and off ~3 times. It ended up fixing it since the child lock internally tripped for reasons unknown to me but showed no external signs of it tripped and "disabled" the door. On the Optima I did it on, you need a key or some other object that fits; their child locks aren't "switched" with a lever like Toyota, Honda (or any sane manufacturer, frankly) adds; yes, it makes it easier for a child to disable the lock but most of them wouldn't know what it is for. When I did the parking lot fix I used a Chevy key; that should give you an idea of how little what you use matters as long as it fits the Hyundai/Kia mechanism.

The reason I think it worked is because it reset the linkages in the door and "disabled" the door but left no visible symptoms on the child lock when it does trip internally. So far it's been fine so I suspect Hyundai uses sloppy child lock linkages that go wrong with no clear signs of it until you find yourself stuck back there. Unless you have kids and use them it's something I would "reset" once in a while by toggling it a few times to make sure it's not going to trap you unexpectedly. I'm not sure what Hyundai base they use for the Optima (maybe the Sonata?). All I know it uses a base chassis with the Theta II 2L Turbo and it's the long wheelbase drivetrain.

If anything, the Hyundai/Kia Theta II engine issues necessitating a lifetime longblock warranty and this door problem have done the opposite of what I have been told about high mileage Japanese cars, especially 200k+ mile Toyotas which have proven themselves. The risk of a high mileage Toyota with 200k+ miles failing like a Hyundai/Kia is a lot more in your favor vs a sudden failure at 250k miles that catches you off guard, yet Hyundai/Kia can't make a reliable engine that doesn't self implode due to factory sloppiness and has child locks that unknowingly lock adults in due to bad design. The high mileage is just scary on paper when you have something like Toyotas regularly hitting 200k with near-perfect reliability (outside of blunders like the 2AZ-FE VX30 oil consumption ending many early due to how bad it gets when the mileage is super high). Yes, it's a marginal risk but if I saw a 150-200k mile Toyota for $1-2k AND it ran perfectly I wouldn't be worried about it being "high mileage".

That's not a fix I feel should have been needed. All it does is confirm my longstanding suspicions about Hyundai QC being subpar (especially after the Theta II short block blunder that happened in the Georgia factory) and subsequent lifetime warranty to settle the problem. If that doesn't work you'll need to take the door card off and check everything over; I advised them to do that ASAP since I trust the mechanism as little as you think after that.