The Sign in with Twitter account requires for you to have a Twitter app.
If you didn’t already have a Twitter app before July 24th when we made this announcement, you will have to have an approved developer account to create a new app.
If you did have an app before that date, you should be able to use and edit that app via apps.twitter.com, and if you apply for a developer account, via the new apps dashboard on developer.twitter.com.