I've found I need to apply plugins in the

subprojects

block explicitly (i.e. even though i've applied the java plugin in my root build.gradle.kts with the plugin DSL, i have to apply it again in the subprojects block using

apply { plugin("java") }

otherwise it doesn't let me define any common dependencies (as the testCompile configuration isn't available). So maybe you have to apply the

maven-publish

plugin in your subprojects block for that extension to be visible.

that's generally true - you must apply plugins in each project specifically