I did some research and here's what I found. This issue typically only happens for people who got windows 11 early on to its release.
Corrupted ieframe.dll element – This dynamic link library file (ieframe.dll) is responsible for breaking a lot of things on Windows 11 including the ability to adjust the brightness. If ieframe.dll is to blame for this issue, the good news is you can fix it by opening up an elevated CMD prompt and running a couple of SFC commands meant at repairing this file.
Improper Monitor driver – This typically occurs when the monitor driver is migrated over from an older Windows version. If this scenario is applicable, you can probably fix it by using Device Manager to forcibly update the generic monitor driver to a dedicated equivalent.
After investigating this issue, it’s clear that the two culprits go hand in hand. So when troubleshooting this issue, you will need to take measures against both of these scenarios.
Press the Windows key + R to open a Run dialog box, then inside it you need to type ‘devmgmt.msc’ and press Enter. When you’re prompted by the User Account Control, click Yes to grant admin access.
After the Device Manager appears on your screen, expand the drop-down menu where it says Monitors. Next, right-click on the monitor, then click on Disable device.
Then, once the generic driver has been disabled, right-click on the monitor and click on Properties from the context menu that just appeared.
Inside the properties menu, click on Driver tab located at the top of the screen, then click on Update Driver.
You will have two options to choose from, so choose Search automatically for drivers. After the search is complete, you can close the menu.
After that, open another Run dialog box by pressing the Windows key + R, and type cmd, then press Ctrl + Shift + Enter to open the CMD prompt with administrator privileges. When you’re prompted by the User Account Control window, click Yes to confirm the admin access
Once you’re finally inside the elevated CMD prompt, type the following commands and press Enter after each one to effectively repair the ieframe.dll file and every associated dependency:
chkdsk
prompt
sfc /scanfile=c:\windows\system32\ieframe.dll
sfc /verifyfile=c:\windows\system32\ieframe.dll
assoc
sfc /scannow
After the process is complete, type exit to close the Command Prompt, then the only thing left to do is to reboot your PC. Afterward, you can check to see if you still encounter the brightness not working problem.
At this point, you should be able to modify the screen brightness on Windows 11.