Hi AFRJ and thanks for being a participant in Apple Support Communities.
I see you have some questions about the keyboard brightness on your MacBook Air.
This is controlled by a piece of kit called the ambient light sensor. On a MacBook Air, this is located behind the speaker grill on the right-hand side. There are a few possible reasons this could be happening. Is there a case or anything else that may be blocking the speaker grille (and therefore the sensor) in any way?
If there is nothing physically blocking that sensor, then there are a few things you can try to make sure this is not software related. You'll want to test after each of these.
1) Safe boot the Mac to clear out system caches and give yourself a solid foundation.
Use safe mode to isolate issues with your Mac - Apple Support
2) Reset the System Management Controller (SMC)
How to reset the System Management Controller (SMC) on your Mac - Apple Support
3) Reset the NVRAM
Reset NVRAM or PRAM on your Mac - Apple Support
If none of these steps return functionality of the sensor, then try testing in a fresh user account as a final isolation step.
How to test an issue in another user account on your Mac - Apple Support
If the above steps are not effective and the issue persists in a test user account, it points to a failure of the sensor and, at that point, you'd want to explore service options in your region.
Apple - Find Locations
Cheers.