Unable to locate package ros-melodic-tf2-tools

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Following this tutorial: http://wiki.ros.org/tf2/Tutorials/Int...

I tried to install tf2_tools and got the following:

sudo apt-get install ros-$ROS_DISTRO-turtle-tf2 ros-$ROS_DISTRO-tf2-tools ros-$ROS_DISTRO-tf
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-melodic-tf2-tools

I tried searching for a solution, but couldn't find anything. Similar posts for kinetic and lunar were not helpful.

Thanks in advance.