MacBook Air (2020 Backlit Keyboard Issue

Since you're still under warranty, there's little you can do by yourself.

First try resetting SMC and NVRAM

How to reset the SMC of your Mac - Apple Support

Reset NVRAM or PRAM on your Mac - Apple Support

If it doesn't work, last action you can take is to reinstall macOS to see if it lies on the software (have a backup your personal data, please)

How to reinstall macOS - Apple Support

Other than that, warranty. Take your Mac to an Apple Store or Authorized for proper diagnostic and further actions.

I also had this issue until I worked out that the Keyboard Brightness can be altered within the Control Centre. Mine was set to zero brightness (possibly because I had upgraded from a MacBook Pro that had the physical Keyboard Brightness key in the Function keys). Check that setting first before making a trip for warranty repair.

Am not sure why the keyboard brightness control goes missing in my control centre. Re-install the MacOS and it became fine but a few weeks later goes missing again.