Ubuntu 18.04 no tweak tool

That's because Ubuntu 18.04 ships by default with GNOME 3.28, in which they renamed the gnome-tweak-tool to gnome-tweaks. And in the Activities Overview can be found by searching for Tweaks.

If it's not installed then you should be able to install it with:

sudo apt-get update
sudo apt-get install gnome-tweaks 

This is due to Canonical packaging it up like this in order not to confuse users too much with the naming change.

Since @WinEunuuchs2Unix asked, here's the answer based on @PRATAP help:

sudo apt update;
sudo apt upgrade -y;
sudo apt install gnome-tweaks;

The upgrade part is very important because otherwise the changes from update won't get installed.