If, while trying to link your account to Google Play Games or Game Center, you are seeing the message that another account is already connected, it means that your current game progress is connected to the Google Play Games / Game Center account different from the one that is currently active on your mobile device.
To make it clearer, let's say your progress was connected to the xxxx@example.com account, and now another (yyyy@example.com) account is selected on the phone (or tablet). You get this error message only because a progress cannot be connected to two accounts at once. To connect progress to yyyy@example.com, first, in the "Accounts" / "Apple-ID" section of the general device settings, you need to log in to xxxx@example.com, return to the game and disconnect your progress from xxxx@example.com in game settings.

If, when you are trying to connect your account to Google Play Games or Game Center, you are seeing the error message that you could not get a response from the server, please try again after a while.

In the case of other types of devices and app stores, the following steps are similar.