This problem may be caused by two reasons:
1. The input software package does not match the ros version.
For example, if I have a 20.04 system and the ros version is noetic, then an error will be reported if I enter this line to prepare the installation instructions for 1804 melodic:
sudo apt-get install ros-melodic-plotjuggler
2. No ros package source added
The solution is very simple. For reason 1, just change to the corresponding ros version. For example, change the melodic to noetic:
sudo apt-get install ros-noetic-plotjuggler
For reason 2, you need to add the installation source:
echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/ros-latest.list
sudo apt update
After adding it, run the installation command.