Toyota Sienna 2018 LE won't start

If it says check charging system then have a shop check it. Now as this has a computer controlled charging center unhooking a battery cable with the engine running can and will fry electronics. I had a customer that did the exact same thing you did and it fried every electronic component the vehicle had. Back then it was well over 3 grand to get it running again, these days probably double that if you can get parts.

Back when cars had generators, you could disconnect the battery too check charging. With alternators, I advise you never disconnect a battery or charge conductor when the engine's running. Spikes can be devastating to electronics.... In your case, you likely just had a defective alternator or voltage regulator. Now, it's likely you have an entire electrical system that needs work. Terrible thing to do to a five year old Toyota. A couple hundred bucks paid to a mechanic could have saved you thousands in repairs.