Need advice- 2020 KX450 cuts out, won’t start, starts, difficulty idling

That sounds bad or very bad. Sorry to say. First check your oil; Level, colour, smell.

Good luck.

Did the bike make any weird noise before it cut out. Bad gas?

What does the oil look like now?
Milky, metal flakes?

There are not a lot of things that can be ruled out at this point. Did it happen again? What did the engine do when it's stopped?

Is it getting hot?? There’s a sensor on the battery that will prevent it from turning over while really hot. But it has to be getting hot from something else if that’s the case.

And if it’s getting lugged around at low speeds, you’re better off with a 1.8 cap.

Wish I could help, but that’s all I’ve got

https://m.youtube.com/watch?v=1j9K5mveQnU

This sounds to me like some form of ECU/Electronics issue. Check your connections.

Can you turn the engine over by putting it in gear and pushing it or pulling the plug in front/above the shifter and turning the rotor bolt/nut? I’m just trying to rule out something mechanical locking it up so the starter won’t turn it over. It might not be a bad idea to pull the valve cover and check the cams where they run in the head.
If it’s not mechanical check the charging system to see if it’s pulling your battery down.

Check for a cracked subframe or another bad connection along the ground somewhere. Someone had a similar issue here with his CRF, turned out the subframe was cracked in 2 locations and the ground was losing contact because of it. He did have previous crash damage to the subframe though that was the original cause of the cracks starting.

Did you happen to wash the bike prior to this happening? My 17 Honda cut out in the middle of a race. After a day or two, it would start and run fine. Turned out to be moisture in the map switch/kill switch. I installed a new switch and it happened again. Even being carefully not to blast the bars with high pressure water, this happened. I was at a race in 2018 and saw the Geico guys wrap the controls with a plastic bag prior to washing it, so this must be a known problem. Different bike brand, but something to eliminate.

Sounds like either a fuel or an electronics issue or maybe even a pressure problem of some sort. I'd look into those areas first off. Yup...

Pull the kill switch and try?

So what was it??

Check the TPS. My 2019 was hard to start, idled poorly, etc. until I set it to factory spec.

Hate to tell you this but my buddy who had a 2020 KX450 had the same problem and it was the oil pump & crank. His bike had the same symptoms as yours. Starts, after a while it cuts out and won't start. The electric start wouldn't kick the bike over. Let it sit for 20min and it would fire then 5 min later do the same thing. The engine usually stops with a large "tick" sound when it dies. Just immediately stops. I was in the pits next to him when it happened the second time after the dealer fixed it the first time. Came rolling into the pits, the bike would barely idle. The electric start would tick but didn't turn the bike over. After the bike cooled down, it started up and would cut out and bog then finally immediately stopped. Not sure if his oil pump failed again, but he had this same problem on the bike with less than 10hrs on it from when he bought the bike brand new off the floor. He ended up selling the bike after the dealer fixed it twice under warranty.

Does the dealership do this?

seems like an obvious electrical connection issue. I would take it to the dealer that you bought it from and have them check it out. It could be so many different things causing this.