To use this action, you must include a copy of the plantuml.jar file within the root directory of your repository.
You can get this file here at: https://sourceforge.net/projects/plantuml/files/plantuml.jar/download
Or run this curl command in your root directory:
curl -o plantuml.jar https://iweb.dl.sourceforge.net/project/plantuml/plantuml.jar
This action works by recursively searching for PlantUML files with the extension .pu in a subdirectory named src.