Ubuntu 20.04 on iMac mid 2011 - Can't Adjust Brightness
Try to add acpi_backlight=vendor as a boot parameter.
Run in a terminal:
sudo nano /etc/default/grub
and add the parameter like this (edit the line):
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Then save the file, run sudo update-grub and reboot.