The key is to enable the device you have under the Monitors section in Device Manager.
This simple method was suggested by a user on a forum and many other users were able to benefit from this method even though it appears to be totally unrelated to the problem. Solution 1: Enable a Device in Device Manager
Old graphics drivers – Having old and outdated graphics card drivers can also provide instability when it comes to brightness settings and we strongly recommend you install the latest drivers as soon as possible.Monitor driver disabled in Device Manager – If this driver is disabled, Windows switches to older versions which don’t provide the capability to change the brightness in that manner.
