Comment 1 for bug 1868566

Grub Customizer runs grub-mkconfig and parses its output. Usually its output is the same as /boot/grub/grub.cfg, but in some cases it may differ (especially when the file was edited by hand or when not up-to-date).

So first take a look at the output of this command (just run `sudo grub-mkconfig`. It wont change your system just printing the new config to your console). Check it for the correct os names.