Charcoal
When the sun gets small, you need to heat up by throwing some big horns in the fire. This turns into charcoal. Nice, toasty charcoal that keeps you warm when the sun gets small.
An update is needed before you can continue but you have more than one WaniKani tab open.
Finish up what ever you are doing on the other tab(s), then close them to continue.
The update will complete automatically once the other tab(s) are closed.