It means that your Google Play account is already connected to a game account. In order to protect yourself from losing the current progress, we recommend that you disconnect your previous progress and connect your new one. To do this:
1. Open the game on another device (emulator or another phone).
2. Restore the game from Google Play.
3. Disconnect your account in the game settings and close the app.
4. Open the game on your main device and connect your current progress to Google Play.
Perform these steps in this strict sequence! After this, you’ll be able to continue playing on your device with connected progress.

On iOS devices, the actions are similar:
1. Open the game on another iOS device (the same Apple ID must be selected on the second device as on the first one)
2. Restore the game from Game Center.
3. Disconnect your account in the game settings and close the app on the second device.
4. Open the game on the first device and connect your current progress to Game Center.

In the case of other types of devices and app stores, the following steps are similar. You’ll need a second device with the same app store and operating system as on the first device. On both devices, you must select the same account to which the game is connected. Thus, by downloading the progress offered by the game, but on a different device, and disconnecting it in the game settings, you’ll be able to connect your main progress on the first device.