Please be sure to source the ROS2 workspace in the same terminal as the one in which you are configuring, compiling and using the plugin. If your installation is system-wide as described in ROS2 Install Instructions:
source /opt/ros/$ROS2_VERSION/setup.bash
You can add this line to your .bashrc
In order to find the sofa lib for out of tree compilation, the environement variable CMAKE_PREFIX_PATH needs to be defined to point to the location of the library you want to use :
export SOFA_INSTALL_DIR=path_to_sofa_install_directory/lib