I have installed Doxygen on Ubuntu 20.04, and I could find the version info by "doxygen -v" in terminal, and I had activated Doxygen in Tools-->PLugins-->Doxygen Integrator, but each time when I opened MPLAB, it would be prompt in "Output --> Doxygen Integrator," :
 
 Doxygen Plugin Information
 Before generating documentation for the project, please download and install doxygen exe from [http://sourceforge.net/projects/doxygen/files/]