When there is a mismatching version in your 'Podfile' against your Xcode Version, it would cause this issue. For example:
My Xcode Version is 9.2 (see below)
Therefore my Podfile will need to include 9.2 in the platform section (see below on line 2)
After that, go back to your Terminal and recreate the '.xcworkspace' file by executing in the project folder:
pod install
Finally, open your '.xcworkspace' file in Xcode and click Build.
This should resolve the issue