The site administrator cannot be deleted
Admin user delete returned The site administrator cannot be deleted. (theSiteAdministratorCannotBeDeleted) when targeting the protected site admin account.
Why this happens
Safety rule: the primary administrator must remain so the site is not locked out. Bulk delete also refuses this account.
Diagnose and fix
1. Deselect the site administrator from bulk delete.
2. Delete or suspend other users instead.
3. Transfer ownership/roles only via supported admin tools — do not remove the last admin.
4. See First admin login for the install-created admin account.