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You cannot apply a plugin that is built by one project of a build to another project in the same build. Put the plugin in its own build and then build a composite build with it, then you can apply it.

@Vampire thank you! That was the term I needed, the composite builds doc page has exactly this scenario too https://docs.gradle.org/current/userguide/composite_builds.html#included_plugin_builds . You saved me a headache...hope you have a good weekend