For my solution, I had changed the Flutter channel to Stable. Then, flutter clean -> pod install -> Clean Build Folder and Archive again. It worked well.

We can debug with iOS 14.6 in Xcode 12.4. Go to this repo and download the iOS 14.6 package, then unzip Open the Applications, Right-click on Xcode 12.4, and click on Show Package Contents. Then, open the DeviceSupport folder by following this path "Developer > Platforms > iPhoneOS.platform" Drag your iOS 14.6 package to this folder, restart your Xcode and build. This solution worked well for me, and I hope it will work for you as well.