Invalid `Podfile` file: cannot load such file. /flutter/packages/flutter_tools/bin/podhelper

When I tried to test flutter project with a simulator after upgrading flutter, this error came up.

Launching lib/main.dart on iPhone 11 Pro in debug mode...
Running pod install...                                              1.1s
CocoaPods' output:
↳
    [!] Invalid `Podfile` file: cannot load such file -- /Volumes/WD /DEVELOPER/flutter/packages/flutter_tools/bin/podhelper.
     #  from /Volumes/WD/DEVELOPER/dont_forget/ios/Podfile:26
     #  -------------------------------------------
     #
     >  require File.expand_path(File.join('packages', 'flutter_tools', 'bin', 'podhelper'), flutter_root)
     #
     #  -------------------------------------------
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-core-1.10.0.beta.2/lib/cocoapods-core/podfile.rb:318:in `rescue in block in from_ruby'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-core-1.10.0.beta.2/lib/cocoapods-core/podfile.rb:312:in `block in from_ruby'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-core-1.10.0.beta.2/lib/cocoapods-core/podfile.rb:50:in `instance_eval'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-core-1.10.0.beta.2/lib/cocoapods-core/podfile.rb:50:in `initialize'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-core-1.10.0.beta.2/lib/cocoapods-core/podfile.rb:310:in `new'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-core-1.10.0.beta.2/lib/cocoapods-core/podfile.rb:310:in `from_ruby'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-core-1.10.0.beta.2/lib/cocoapods-core/podfile.rb:276:in `from_file'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.0.beta.2/lib/cocoapods/config.rb:205:in `podfile'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.0.beta.2/lib/cocoapods/command.rb:160:in `verify_podfile_exists!'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.0.beta.2/lib/cocoapods/command/install.rb:46:in `run'
    /Library/Ruby/Gems/2.6.0/gems/claide-1.0.3/lib/claide/command.rb:334:in `run'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.0.beta.2/lib/cocoapods/command.rb:52:in `run'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.10.0.beta.2/bin/pod:55:in `<top (required)>'
    /usr/local/bin/pod:23:in `load'
    /usr/local/bin/pod:23:in `<main>'
Error running pod install
Error launching application on iPhone 11 Pro.
Exited (sigterm)

I tried flutter clean, flutter build iOS, rm iOS/Podfile but it didn't work at all. How can I solve this? Here's my flutter doctor and flutter --version below.

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 1.20.2, on Mac OS X 10.15.5 19F101, locale en-KR)
 
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.1)
[✓] Xcode - develop for iOS and macOS (Xcode 11.6)
[✓] Android Studio (version 4.0)
[✓] VS Code (version 1.48.1)
[✓] Connected device (1 available)

• No issues found!

Flutter 1.20.2 • channel stable • https://github.com/flutter/flutter.git
Framework • revision bbfbf1770c (10 days ago) • 2020-08-13 08:33:09 -0700
Engine • revision 9d5b21729f
Tools • Dart 2.9.1

Run from terminal:

flutter clean

flutter pub get

pod install // cd into ios folder

Run these commands from your project's root directory:

flutter clean
flutter pub get
cd ios
pod install
pod update