"gnome-tweak-tool: command not found" after installation

In Ubuntu 18.04 aka Bionic Beaver the gnome-tweak-tool is a transitional package which is used to pull in a different package, in your case gnome-tweaks to make upgrades more smooth when a package name had to be renamed. For whatever reasons...

You can see this information when using apt-cache show gnome-tweak-tool. The Depends: line is the package that is pulled in.

So to run it, just enter gnome-tweaks or use dpkg -L gnome-tweaks to list the files that are shipped with that package.