When a new update is rolled out, it can happen that the download is not verified by the App Store yet. In such a case, the game server detects an old client version and sends out the "Client Version Outdated" error message asking the player to update. As the update is not released on the App Store yet, the update cannot be installed, and the game seems to be up to date. Should this happen, please wait a couple of hours and the update will be released to the App Store as soon as possible.
Should the issue persists, please try uninstalling and reinstalling the game.