Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / modules / contrib / metatag / metatag_hreflang / config / schema / metatag_hreflang.metatag_tag.schema.yml

# The 'type' should be "label" for short meta tags and "text" for ones which
# could get longer, especially ones which use a textarea field instead of a
# textfield.

metatag.metatag_tag.hreflang_xdefault:
  type: label
  label: 'Default locale (x-default)'