This query is working fine, but Idea complains that it "Cannot mix collection and single variables":

        String q = "select scoreEntry.department.id, scoreEntry.department.name, scoreEntry.yearMonth, sum(scoreEntry.score) " +
                   "from ScoreEntry scoreEntry " +
                   "where scoreEntry.yearMonth >= :startMonth and scoreEntry.yearMonth <= :endMonth " +
                   "group by scoreEntry.department.id, scoreEntry.department.name, scoreEntry.yearMonth";

Issue was resolved