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.