Cause
Dependencies among schemas (based on a variety of conditions) determine whether un-versioning is allowed.
If a superschema is versioned and the latest version of the superschema was created before the schema was processed, un-versioning is not possible and this error occurs. For example, if a schema is added to a schema that has already been versioned, the versioned schema cannot be unversioned.
Action
As un-versioning is not possible, your only course of action is to carry out the reorganization or to restore your system from backup.