On that model a backlit keyboard was an option. Your laptop may have been ordered without backlit keyboard. On that model, the palmrest/keyboard/touchpad is a single unit. If you now want backlit keyboard, the laptop would have to be disassembled and that unit replaced with one that includes backlit keyboard. I checked Parts-People who have the non-backlit unit in stock, but not with backlight. Maybe you can find it on eBay.
My laptop model is Inspiron 15 3511. Is there a way to enable keyboard light? But there is no icon on the F5 button?
Why is keyboard light not enabled? There is no keyboard lights up icon on F5 
button.