Windows 10 now supports adpatvice screen brightness if the hardware supports it. Sometimes this feature breaks and here are some of the fixes.

• Solved: Brightness option is greyed out - HP Support Forum - 5864386 - Change the monitor driver. The monitor was set to use PNP Monitor, change this to use Generic PNP and this resolved the issue
• How to Adjust Your PC’s Screen Brightness, Manually and Automatically
• Check the graphic settings with any harwarde specifc software that was supplied with the graphics card
• Update the graphics driver
• enable adaptive screen management in the windows power profile
• install the button software - Some laptops will need their own software to access the special buttons such as increase brightness.
• windows 10 screen brightness greyed out - Google Search