WARNING : "SDK Version Issue". xcode 10.1
Make sure you have the latest version of Xcode.
If you do have Xcode 10.1 (or later) this is just a reminder that all of your apps need to comply with these terms.
Also regardless of Xcode version if your app did archive/build successfully you have nothing to worry about at the moment.
EDIT:
Now ALL APPS and GAMES submitted to the AppStore must have the 12.1 SDK or they will be rejected.
If your app is still rejected and you have Xcode 10.1 (or later), update Xcode or reinstall, as the SDK was not integrated correctly by the system.
This trick worked for me:Download Xcode 10.2, extract and just copy(iPhoneOS12.2.sdk Folder):or direct download zip : https://drive.google.com/file/d/1ZGANCBE90Ogt4FD_y_ACgcIFP0qQapSf/view?usp=sharing
Right click->Show Package Contents->Contents->Developer->Platforms->iPhoneOS.platform->Developer->SDKs->iPhoneOS12.2.sdkand past it same directory on your Xcode.after this close your Xcode, reopen Xcode , Archive and upload again.