In general, if it says you cannot delete someone, it is because deleting them would cut-off some people from your tree, effectively cutting off a "branch" of your tree. For example, if you delete the only parent of some children (because you don't know the other parent), deleting that parent would "orphan" those children, and they would no longer be linked to your tree. The system won't allow that.
If you really want to delete the person, you'll need to first delete anyone linked to only that person; could be ancestors, descendents, or partners. Anyone who is only linked to that person. Once there are no more people who only link to that person, then the system will allow you to delete them.
