Hello andresharps,
Thank you for posting in the Apple Support Communities. We understand you're unable to manually adjust the brightness on your Mac because the setting is greyed out, and we'd like to offer a few steps that might help.
First, be sure you're taking the steps below to adjust the brightness. Keep in mind that in macOS 10.10 and later, the Brightness slider may no longer appear in the Display pane of Displays preferences.
Change your Mac display’s brightness
If you haven't already, restart your Mac. Restarting sounds simple, but can quickly resolve many unique issues. From there, switch to another user account, then see if the same issue occurs there. This can help identify whether this could be user-specific, or system-wide. Another way to isolate user settings is to start in safe mode. Here's how:
Start up your Mac in safe mode
We hope this information is useful. Kind regards.