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Try sudo apt-get install ros-melodic-rviz-visual-tools. And then check if it's there with either rospack list-names or rospack find rviz_visual_tools. You can also build it from source by cloning it from the official git.
Alternatively, in your CMakeLists.txt file, under #find_packages or ##System dependencies are found with CMake's conventions, add & save find_package(rviz_visual_tools REQUIRED). Then check with the same rospack commands.