In some forums I found solutions like this:
Seems like the "Cordova" folder isn't getting copied into the project during cordova platform add ios So, you'll have to copy the "Cordova" source folder into your project manually. But you'll have to download the ios source packages from github first, which is located at:
github.com/apache/cordova-ios
After downloading the "cordova-ios-master.zip" package and unzipping it, copy the folder named "CordovaLib" into your project next to the Xcode project file:
PROJECT_FOLDER/platforms/ios/PROJECT_NAME
PROJECT_FOLDER/platforms/ios/PROJECT_NAME.xcodeproj
PROJECT_FOLDER/platforms/ios/PROJECT_NAME.xcworkspace
PROJECT_FOLDER/platforms/ios/cordova
PROJECT_FOLDER/platforms/ios/CordovaLib <------- here
PROJECT_FOLDER/platforms/ios/ios.json
PROJECT_FOLDER/platforms/ios/platform_www
PROJECT_FOLDER/platforms/ios/pods-debug.xcconfig
PROJECT_FOLDER/platforms/ios/pods-release.xcconfig
PROJECT_FOLDER/platforms/ios/www
But it does not work...