The Exception is straightforward and says : Illegal attempt to map a non collection as a @OneToMany, @ManyToMany or @CollectionOfElements, so the cause is obvious here and if we take a look at the Hibernate Collection mapping documentation it clearly states that:
As a requirement persistent collection-valued fields must be declared as an interface type (see Example 7.2, “Collection mapping using @OneToMany and @JoinColumn”). The actual interface might be
java.util.Set,java.util.Collection,java.util.List,java.util.Map,java.util.SortedSet,java.util.SortedMap…
And you used TreeSet which is an implementation class for both Set<E> and SortedSet<E> interfaces. So your actual mapping won’t work with TreeSet, you should use a Set<CoachGroup> instead of a TreeSet<CoachGroup>:
private Set<CoachGroup> coachGroups = new HashSet<CoachGroup>();