On a Mac computer, you need an Apple display attached to control the brightness level with your solar keyboard. If your Apple display has USB plugs, these also need to be attached to your system for the keyboard to control brightness.
In addition, the F-keys labeled for brightness up (F7) and down (F8) won't work while an Apple keyboard is connected along with the solar keyboard. To fix this issue, remove any Apple keyboards and try again.
NOTE: If you have F14 and F15 shortcuts assigned for brightness control, you'll have the same issue.