Keyboard Backlit not working.

Click Control center in the menu bar and try adjusting it.

Reset  SMC:   How to reset the SMC of your Mac - Apple Support   Choose the appropriate method.

Quote:

Resetting the system management controller (SMC) can resolve certain issues related to power, battery, fans, and other features.

Startup in Safe mode. https://support.apple.com/en-gb/HT201262

To leave Safe Mode, click  in the menu bar and choose “Restart”

Quote:

Safe mode prevents your Mac from loading certain software when it starts up, including login items, system extensions not required by macOS, and fonts not installed by macOS. It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. And it deletes some system caches, including font caches and the kernel cache, which will be created again automatically as needed.

Anees_Javaid Said:

"Keyboard Backlit not working.: Keyboard backlit is not working. When I restart my MacBook pro the keyboard backlit illuminates till the login screen. when I log in using touch id or password, the backlit turns off. Backlit buttons on the touch bar are also not working."

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Login Screen Issues...

Reinstall the macOS:

If it is happening at the logIn screen, then you might have to reinstall the macOS. It's just that items that have become corrupted or deleted, would be replaced or relocated back to where they should be, respectively. Go Here: How to reinstall macOS from macOS Recovery - Apple Support

Important:

Create a Time Machine Backup, First:

Prior to the re-installation of your macOS, be certain to backup your Mac using a Time Machine Backup. That way, you can have something to restore your Mac from, should anything go wrong with the reinstall. Use an external hard drive that is at least 1.5 the total storage capacity.

Same here

if u get the solution for this problem please mentions this

I have MacBook Air 2019 same keyboard backlight problem Version 'Big Sur 11.4'