Many people like to play the game on multiple devices. When there is a new update available, it is possible to install it on one device but not update the other(s). As soon as the latest (major) version has been played on one device, the account cannot be used to play on other devices anymore.

Normally this issue can be resolved simply by updating the game to the latest version on the other device(s), too. However, if the game is being played on multiple devices that are using a different platform (Android and iOS), it is possible that the new version is not yet released for the other platform. It is useful to note that the Android version is often the first one available.
If you require help in updating the game on an iOS or Android device, please check out these articles:

It is important to be aware that the update might not be available for all players on both platforms instantly, as it is being rolled out around the globe gradually. Please be patient. If the App Store or Google Play does not have the update yet - it should arrive soon!