When I tried to publish an app with Watch Kit App/Extension, I have the error:

ERROR ITMS-90203: “Invalid architecture: Apps that include an app extension and a framework must support arm64

You can verify with lipo on your binary.

lipo -info "/Users/junda/Library/Developer/Xcode/Archives/2015-04-04/AwesomeApp 4-4-15 8.49 pm.xcarchive/Products/Applications/AwesomeApp.app/AwesomeApp" 

It must list arm64.

If not, make sure all your targets includes arm64 in Build Settings > Architectures and also Valid Architectures.

The standard architecture is armv7 and arm64.

NOTE: It is NOT armv64, it is arm64. A typo will result in not building for arm64 too.

When you build/archive, disconnect any connecting device too, because that could trigger “Build Active Architecture Only”.