Failed to Start Load/save Screen Backlight

Hi!
I think you misplace the option, it should be in the line before, just after udev.log_priority=3. Then sudo update-gub
About the other error, you can try and add in the same line acpi_osi=Linux.
After every edit on the grub file, you must do sudo update-grub.
Also you can take a look at this thread