You are not defining a repository in your build. For many people that would be Maven Central.
repositories {
mavenCentral()
}
It seems as if you want to manage your libraries yourself as you are pointing to the lib
folder. I assume these libraries are checked in with your source code? If the same strategy should apply to the JaCoCo libraries then you will need to put them there and assign them to the configurations of the JaCoCo plugin.