In the OptaPlanner 8.27.0.Final examples, the IdleCost class has a wrong package defined: org.optaplanner.examples.cheaptime.optional.score while it is stored in the 

org/optaplanner/examples/cheaptime/optional/score/drools folder.

This class only seems to be used by the cheapTimeConstraints.drl file, and a quick fix is to move the class one folder up.