创建引导macOs安装盘出错

CindydeMacBook-Pro:~ cindy$ sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Password:

/Applications/Install macOS High Sierra.app does not appear to be a valid OS installer application.

CindydeMacBook-Pro:~ cindy$ /Volumes/MyVolume 

U盘为GUID分区,名称为MyVolume。

在应用程序里有macOs high Sierra 的安装程序。可是做u盘安装盘就说找不到程序。求解，谢谢！