Xcode compilation error: ld: character (s) not found for x86_64 clang architecture

Question

I am developing an iOS application that had no problems compiling the project. But now, when I try to compile, I get the following error message:

Undefined symbols for architecture x86_64: "_OBJC_CLASS_$_CKCalendarView", referenced from: objc-class-ref in CITSearchCarViewController.o objc-class-ref in CITAddNewDestViewController.o objc-class-ref in CITSearchViewController.o objc-class-ref in CITNewFlightAlertViewController.o "_OBJC_CLASS_$_HMSegmentedControl", referenced from: _OBJC_CLASS_$_CITSegmentedControl in CITSegmentedControl.o "_OBJC_CLASS_$_IBCoreDataStore", referenced from: objc-class-ref in CITSearchHistory.o objc-class-ref in CITJourneyData.o objc-class-ref in CITAlertManager.o "_OBJC_CLASS_$_MKNetworkEngine", referenced from: _OBJC_CLASS_$_CITNetworkEngine in CITNetworkEngine.o "_OBJC_CLASS_$_SSZipArchive", referenced from: objc-class-ref in CITUtils.o "_OBJC_CLASS_$_SVProgressHUD", referenced from: objc-class-ref in CITInitialViewController.o objc-class-ref in CITFlightAlertViewController.o objc-class-ref in CITSearchViewController.o objc-class-ref in CITSearchResultsViewController.o objc-class-ref in CITMultiDestViewController.o objc-class-ref in CITConfigurationViewController.o objc-class-ref in CITFlightDetailViewController.o ... "_OBJC_CLASS_$_TRAutocompleteView", referenced from: objc-class-ref in CITAddNewDestViewController.o objc-class-ref in CITSearchViewController.o objc-class-ref in CITNewFlightAlertViewController.o "_OBJC_METACLASS_$_HMSegmentedControl", referenced from: _OBJC_METACLASS_$_CITSegmentedControl in CITSegmentedControl.o "_OBJC_METACLASS_$_MKNetworkEngine", referenced from: _OBJC_METACLASS_$_CITNetworkEngine in CITNetworkEngine.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) 

I tried changing the architecture of i386 to x86_64, but I get the same result. The problem was originally caused by Xcode not finding the lpods library. Then I tried to “clean and build” the project, but it works.

What can I do?