Xcode 9 doesn't have any simulator device
Go to Xcode Preferences -> Locations and go to the Derived Data location in Finder:
Just remove all contents from it, then right click on Xcode icon and quit it, and reopen it:
It is a simulator but you need to close Xcode 8 if you have it open and restart Xcode 9 beta.
This may happen due to multiple Xcode installed on machine or might be the deployment target is higher than the simulator OS version.
In order to resolve this navigate to following location : Xcode > Preference > location > comindline Tool > change the xcode version
and decrease the deployment target.