Hi Danny-Ali,
Thanks for contacting Apple Support Communities!
To confirm, are you able to go to Settings > Accessibility > Display & Text Size, and toggle the button here? Auto-Brightness will be at the bottom of the page in Display & Text Size: Adjust the screen brightness and color on iPhone - Apple Support
"Adjust the screen brightness automatically
iPhone adjusts the screen brightness for current light conditions using the built-in ambient light sensor.
- Go to Settings
> Accessibility. - Tap Display & Text Size, then turn on Auto-Brightness."
When using the Search feature in Settings app, tapping on Auto-Brightness will only bring you to the Accessibility Settings location for this. You still have to manually toggle the button itself.
Do you have any profiles or VPN software installed on your iPhone? You can go to Settings > General to check. If so, you may consider disabling any VPN or profiles on your iPhone. Some features of the iPhone may be restricted if using a profile.
If you're still unable to toggle Auto-Brightness, and you haven't done so already, restart the iPhone. Sometimes a restart can help resolve issues.
Best regards.