AssertionError: CSS files should be specified as key/value pairs, where the values are configuration options.

Remove version key from css:

datetimepicker:
  version: 1.x
  js:
    js/moment.js: {}
    js/moment-with-locales.js: {}
    https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js: { type: external, minified: true }
    js/bootstrap-datetimepicker.js: {}
    js/drupal.bootstrap_datetime.js: {}
  css:
    <del>version: 1.x</del>
    theme:
      https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css: { type: external, minified: true }
      css/bootstrap-datetimepicker.css: {}
  dependencies:
    - core/jquery
    - core/drupal
    - core/drupalSettings
    - core/jquery.once