This happened to me when I had two versions of Xcode running on the same machine.
The Xcode-select value could be pointing to the wrong version. (For example, if you're running Xcode 9, but it's pointing to an Xcode 8 version that's also installed on your machine, you might run into this)
If this is your issue, then try changing the value to the right directory by using something like this:
sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
This also, however, happens if files have incorrect permissions or a library/developer file has been manually deleted.
If you have only one version of Xcode installed, try deleting the derived data of the project and force-closing + restarting Xcode, or even reinstalling the IDE. More on deleting derived data here.
Let me know if any of this was helpful!