Illegal Configuration: Compiling IB documents for earlier than iOS 7 is no longer supported

XCode 9 will not support storyboards that are built for iOS version less than 7.

Try changing the Builds for in File Inspector to a higher version then Clean and Build.

See screenshot

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Edit: Important info from @user435779 below: Restart XCode if the errors don't go away.

Simply set the deployment target to 8.0 + in General to fix this issue. No need to go through each xib file.

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As storyboard in Xcode 9 will not support which is below iOS 7.0, so you need to update the iOS version in File inspector.

For more information please find the below GIF representation.

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Hope it works!!!

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