There are a few reasons you might see this error message when trying to create or join a co-op match:
1. You or your co-op partner may not be running the latest version of the game, so please check the app store relative to your device's operating system, refresh the update list and ensure you've downloaded the most up to date version!
2. You or your co-op partner may have downloaded the game from an unofficial source. Please double check that the game has been downloaded from the official store of your device's operating system.
3. You or your co-op partner's save or executable may have been modified. If the resolution in Step 1 doesn't help, please submit a support ticket with you and your co-op partner's user IDs then one of our support team will look into it for you!