This action could not be completed. Try Again (-22421)

Just try exporting the iPA file and then upload that exported iPA file with application loader. It will solve your problem.

(As of May 23, 2018)

Step 1

Open Xcode --> Product tab --> Archive --> export and save to desktop

Step 2

Open Xcode --> Xcode tab --> Open Developer Tool --> Application Loader >> double-click Deliver YourApp and select the recently exported .ipa file from your desktop

Solved the issue every time =)

There's another way to fix above error. Try this, it fixed it for me. Open Terminal and run:

cd ~  
mv .itmstransporter/ .old_itmstransporter/  
"/Applications/Xcode.app/Contents/Applications/Application Loader.app/Contents/itms/bin/iTMSTransporter" 

Three above code line will update iTMSTransporter, then you can try uploading in XCode again.

Refs for more details: https://forums.developer.apple.com/thread/76803

This happens because Apple's servers may not be working correctly. Just wait and try later or next time. It will work for sure eventually.