EDIT

There are PKN left that are linked to an existing key but have no parent but link to a taxon (so are no root nodes).

SELECT *
FROM PolytomousKeyNode pkn
WHERE pkn.parent_id IS NULL
AND pkn.taxon_id IS NOT NULL

Check carefully that they are not linked as parents and that they are not root nodes. Maybe run recursively until none is left.

Also use the script used in #6226 to remove orphaned KeyStatements afterwards.

There are 16 examples in cyprus.