In order to create the Mask output, the plugin looks for an HDRenderPipelineAsset in the Project Settings. You will need to add the plugin to an existing HDRP project. The 2.4.0 version does not support upgrading to HDRP projects.
In the 2.4.0 update, we introduced support for Unity's HDRP scriptable render pipeline. The plugin will automatically create the Mask output required by the HDRP shaders and the generated Unity material is set to HDRP/LitTessellation.