Error message:
Undefined symbols for architecture arm64:
  "_OBJC_CLASS_$_GULAppEnvironmentUtil", referenced from:
      objc-class-ref in FirebaseCore(FIRBundleUtil.o)
  "_GULResetLogger", referenced from:
      _FIRResetLogger in FirebaseCore(FIRLogger.o)
  "_GULLoggerRegisterVersion", referenced from:
      ___FIRLoggerInitializeASL_block_invoke in FirebaseCore(FIRLogger.o)
  "_GULLoggerInitializeASL", referenced from:
      ___FIRLoggerInitializeASL_block_invoke in FirebaseCore(FIRLogger.o)
  "_GULLogBasic", referenced from:
      _FIRLogBasic in FirebaseCore(FIRLogger.o)
  "_GULLoggerEnableSTDERR", referenced from:
      ___FIRLoggerInitializeASL_block_invoke in FirebaseCore(FIRLogger.o)
  "_GULIsLoggableLevel", referenced from:
      _FIRIsLoggableLevel in FirebaseCore(FIRLogger.o)
  "_GULSetLoggerLevel", referenced from:
      _FIRSetLoggerLevel in FirebaseCore(FIRLogger.o)
  "_GULLoggerForceDebug", referenced from:
      ___FIRLoggerInitializeASL_block_invoke in FirebaseCore(FIRLogger.o)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
And I used cocoapods
# Uncomment the next line to define a global platform for your project
# platform :ios, '9.0'
target 'drivethru' do
  # Comment the next line if you don't want to use dynamic frameworks
  use_frameworks!
  # Pods for drivethru
  pod 'Firebase/Core'
  pod 'Firebase/Database'
  pod 'SVProgressHUD'
 pod 'Firebase'
end