xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

[YongilChoi]

$ pod install
Analyzing dependencies
Downloading dependencies
Installing Alamofire (5.9.1)
Installing AppAuth (1.7.5)
Installing CocoaLumberjack (3.8.5)
Installing FSCalendar (2.8.4)
Installing Firebase (10.25.0)
Installing FirebaseAnalytics (10.25.0)
Installing FirebaseCore (10.25.0)
Installing FirebaseCoreExtension (10.25.0)
Installing FirebaseCoreInternal (10.25.0)
Installing FirebaseCrashlytics (10.25.0)
Installing FirebaseInstallations (10.25.0)
Installing FirebaseRemoteConfigInterop (10.25.0)
Installing FirebaseSessions (10.25.0)
Installing GTMAppAuth (1.3.1)
Installing GTMSessionFetcher (1.7.2)
Installing GoogleAppMeasurement (10.25.0)
Installing GoogleDataTransport (9.4.1)
Installing GoogleSignIn (6.0.2)
Installing GoogleUtilities (7.13.3)
Installing MQTTClient (0.15.3)
Installing Moya (15.0.0)
Installing PromisesObjC (2.4.0)
Installing PromisesSwift (2.4.0)
Installing SwiftGen (6.6.3)
Installing WebViewJavascriptBridge (6.0.3)
Installing hpple (0.2.0)
Installing nanopb (2.30910.0)
Generating Pods project

xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

=> How to solve this problem?