This particular error refers to the rule about STRUCTURAL objectclasses, which states that an object is of one STRUCTURAL class, the structural class of the object. The object is said to belong to this class, zero or more auxiliaries classes, and their superclasses. While all of these classes are commonly listed in the objectClass attribute of the entry, one of these classes is the structural object class of the entry. Thus, it is OK for an objectClass attribute to contain inetOrgPerson, organizationalPerson, and person because they inherit one from another to form a single superclass chain. That is, inetOrgPerson SUPs organizationPerson SUPs person. On the other hand, it is invalid for both inetOrgPerson and account to be listed in objectClass as inetOrgPerson and account are not part of the same superclass chain (unless some other class is also listed with is a subclass of both).
To resolve this problem, one must determine which class will better serve structural object class for the entry, adding this class to the objectClass attribute (if not already present), and remove any other structural class from the entry's objectClass attribute which is not a superclass of the structural object class.
Which object class is better depends on the particulars of the situation. One generally should consult the documentation for the applications one is using for help in making the determination.