I would recommend using the excellent third party tool Grub Customizer, which can be installed with:
sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt update
sudo apt install grub-customizer
# To remove
# sudo apt remove --autoremove grub-customizer
Specifically, to control the resolution:
Open the program (type "grub customizer" in ...
Such programs are sometimes called "Launchers" or "Program launchers" or "Application launchers". Some more launchers are Kupfer, Albert, Ulauncher. Rofi is a launcher that initially expanded on dmenu, but that, besides being able to substitute for the latter, has a lot possibilities on its own.
You will probably have more hits and inspiration looking on ...
@Gryu and I had a discussion about what could wrong with this. We tried several things. However, what fixed was accepted answer from How to fix Unity locked disabled settings? even though Ubuntu 18.04 doesn't have Unity.
I typed the command:
mv ~/.config/dconf/ ~/.config/dconf.bak
and all the lock were disabled upon reboot, I was able to change the ...