I know this is already answered, but for those who find this thread in the future there is an official link addressing this issue:

Xcode 12.5 troubleshooting guide (RN 0.61/0.62/0.63/0.64)

There is an open RN issue here: https://github.com/facebook/react-native/issues/31179

For me commenting out Flipper in the Podfile, pod install, and rebuild worked as a temp solution.

  # Enables Flipper.
  #
  # Note that if you have use_frameworks! enabled, Flipper will not work and
  # you should disable these next few lines.
  # use_flipper!
  # post_install do |installer|
  #   flipper_post_install(installer)
  # end

There's a solution I found here.

Add this to your post_install in your Podfile:

post_install do |installer|
  flipper_post_install(installer)

  ## Fix for Flipper-Folly on iOS 14.5
  find_and_replace("Pods/Flipper-Folly/folly/synchronization/DistributedMutex-inl.h",
    "atomic_notify_one(state)", "folly::atomic_notify_one(state)")

  find_and_replace("Pods/Flipper-Folly/folly/synchronization/DistributedMutex-inl.h",
    "atomic_wait_until(&state, previous | data, deadline)", "folly::atomic_wait_until(&state, previous | data, deadline)")
end

You'll also need to add the function def for this find_and_replace function (you can put this function anywhere in the podfile):

def find_and_replace(dir, findstr, replacestr)
  Dir[dir].each do |name|
      text = File.read(name)
      replace = text.gsub(findstr,replacestr)
      if text != replace
          puts "Fix: " + name
          File.open(name, "w") { |file| file.puts replace }
          STDOUT.flush
      end
  end
  Dir[dir + '*/'].each(&method(:find_and_replace))
end

Run pod install again and it should work. If you get an error relating to permissions while accessing the DistributedMutex-inl.h file, delete your /pods folder and run pod install again

You should see text print out that says Fix: Pods/Flipper-Folly/folly/synchronization/DistributedMutex-inl.h, etc. If you don't, double check the path in the find_and_replace call.

just comment out the flipper line in a pod and in Xcode also if you are not using it, hope this will fix in future updates.

# Note that if you have use_frameworks! enabled, Flipper will not work and
# you should disable the next line.
# use_flipper!({ 'Flipper' => '0.79.1'})
#   post_install do |installer|
#     react_native_post_install(installer)
#   end

IN Xcode comment all import and flipper variable

Solution without losing Flipper functionality:

Define the upgraded dependencies for Flipper in the Podfile

React Native 62

def add_flipper_pods!(versions = {})
  versions['Flipper'] ||= '~> 0.87.0' 👈
  versions['DoubleConversion'] ||= '1.1.7'
  versions['Flipper-Folly'] ||= '~> 2.5.3' 👈
  versions['Flipper-Glog'] ||= '0.3.6'
  versions['Flipper-PeerTalk'] ||= '~> 0.0.4'
  versions['Flipper-RSocket'] ||= '~> 1.3.1' 👈

React Native 63

# Enables Flipper.
  #
  # Note that if you have use_frameworks! enabled, Flipper will not work and
  # you should disable these next few lines.
  use_flipper!({'Flipper' => '0.87.0' 👈, 'Flipper-Folly' => '2.5.3' 👈, 'Flipper-RSocket' => '1.3.1' 👈})
  post_install do |installer|
    flipper_post_install(installer)
  end

cd ios && pod install and you should be good to go

React Native 64

Bump the version to 0.64.1 in the package.json

yarn install && pod install --repo-update && react-native run-ios

https://github.com/facebook/react-native/releases/tag/v0.64.1

React native 0.64.1 has been released and will solve this. Just update the version number in package.json.

"react-native": "0.64.1",

No need to modify the Podfile if you do this

Upgrade to react native 0.64.1 (or higher)

if you moved from 0.63.x to 0.64.x make sure that in your Podfile you've updated the use_flipper! section post install:

Before:

post_install do |installer|
  flipper_post_install(installer)
end

After:

post_install do |installer|
  react_native_post_install(installer)
end

run 'npx react-native-clean-project' from project root then from inside './ios' folder run 'pod install --repo-update' then maybe run 'pod update'

use_flipper!({ 'Flipper-Folly' => '2.5.3', 'Flipper' => '0.87.0', 'Flipper-RSocket' => '1.3.1' })

that works, ;)