car won't start

This only started yesterday, but when I push the start button I get a message that says "key is not detected". After I try a few times it will sometimes start, but not always. What is going on?

Most likely one of these:
- key fob battery is dead or low. If you've seen https://imgur.com/a/ff7C6w0 in the dash, it's the warning telling you the of this.
- interference/something blocking communications between the fob and car (e.g. phone in the same pocket/in the way)
- less likely: low 12 volt battery

Try holding up the key fob to the power button and starting, the next time. You should hear a chime as the fob is almost up to the button.

What model year Leaf do you have? The above indicator applies to '11 to '17 Leafs. I don't know off the top of my head the equivalent indicator on '18 to '19 Leafs.

It's a 2016. I don't get a battery fob low signal. It can't be interference because I will hold the key right next to the start button. It was working fine until yesterday when it happened the first time. I just went out and the car started right up.

I suspect the battery in the key fob since the car is at least 2 years old. The dash display should warn you, but loose connections at the little battery or a marginal battery can cause the voltage to drop suddenly and prevent transmission (or greatly reduce the transmit power). I generally need to replace the fob battery after about 2 years on the one I use daily. The one that stays in the desk drawer lasts longer.