My L90 has no auto-brightness option. Could anyone please suggest a way to fix the issue?
The L90 lacks an ambient light sensor, so there is no true auto brightness method.
XDA user gdjindal has a unique tutorial for adding a pseudo-automatic function on the Optimus L9 (it'll most likely work on your phone as well): link
Other option is to use my app. It is real auto brightness. "Auto Brightness by PP"