yarn initをおこなうと以下のようなエラーが出た。

$ yarn init
yarn init v0.18.1
error An unexpected error occurred: "Can't answer a question unless a user TTY".
info If you think this is a bug, please open a bug report with the information provided in "C:\\Users\\username\\...\\projectname\\yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/init for documentation about this command.

以下で解決した。

$ yarn add yarn
$ node ./node_modules/yarn/bin/yarn.js init

参考

https://github.com/yarnpkg/yarn/issues/743