If you are seeing a "Skillz Activation Failed" or an "Invalid Client Serve Request" (on Android) error when you start your application it means that your binary is unregistered in the Skillz system.
For details about game binaries and fingerprint please refer to the Game Binaries documentation.
If you have already uploaded your binary in the Skillz Developer Portal and are still seeing this error there a few things you should check in your troubleshooting process.
The most common reason for the error is testing your game in the iOS simulator. Any game running in the simulator will have a different unique fingerprint, separate from the binary you uploaded. Because of this, you can only test Skillz production mode on an actual iOS device. You can use the XCode Organizer to load the binary you gave to Skillz on to your iOS device, and then test from there.
It is also possible that you are not using the exact binary that you uploaded to the Skillz system. If you created a new archive in Xcode, the fingerprint will be different and will not match the one in the Skillz system. The fingerprint for the current binary and architecture can be found in the Skillz debug screen for iOS or Android. That fingerprint should be present and enabled in the Skillz Developer Portal.