Why does simpletest report a missing schema for my Drupal 8 module?
I see several errors, not sure which one trips you: you missed a type for riddles and tried to define a mapping immediately under. But riddles is a sequence and each element in the sequence is that mapping. So:

riddler.settings:
  type: config_object
  mapping:
    riddles:
      type: sequence
      sequence:
        type: mapping
          question:
            type: label
            label: 'Question'
          response:
            type: label
            label: 'Response'


At least that's what I would attempt...