Sometimes after Ubuntu update or after the installation and uninstallation of some packages, you are not able to find the Setting options (Gnome control center) in your Ubuntu OS. To fix this settings app missing problem just follow the below steps.
First check if the settings app installed on your system. In terminal (ctrl+alt+t) run below command.
If you get the not found error then install the settings app using below command.
sudo apt install gnome-control-center
Enter your Ubuntu password and confirm the installation during the install process. After the installation you can run below command.
You can also search it via the show applications menu.