How do I fix this error?
Change the tweet.info.yml file as follows.

name: 'tweet'
description: 'A twitter Bootstrap starter theme'
core: 8.x
type: theme
base theme: false

libraries:
  - 'tweet/bootstrap'


Put the following in the tweet.libraries.yml file.

bootstrap:
  version: 3.3.6
  css:
    theme:
      css/bootstrap.min.css: {}
      css/bootstrap-theme.min.css: {}
  js:
    js/bootstrap.min.js: {}
  dependencies:
    - core/jquery


Also, be sure you have a tweet.theme file in the same directory. This file is necessary for Drupal to recognize the directory contains a theme.