When you want to use twitter and social feed shortcodes so you need to go your twitter account for generate API keys and access token for work it. To make this, you have to create a Twitter application to generate API Keys, Access Token and secret keys by following some steps:
Go to https://apps.twitter.com/ and click create new
Create an account
After create account, go to Keys and Access Tokens tab
Copy Consumer key, consumer secret, Access Token and Access Token Secret.
Now go to your site admin panel. Then Extensions => plugins => Shortcode Ultimate => API key settings tab => TWITTER SETTINGS and paste the keys.
After fill-up all blank field as per your twitter account and finally click save button.