By default, Plague Inc. will use the account you already have signed in to your Google Play account.
However, you or someone, may have changed this in the past and not remember or there might have been an issue between both apps.
To learn how to find out what is the email address the Plague Inc. App is currently using to sign you in follow the steps:
In the main menu tap the Game Pad button on the bottom left-corner of your screen.
After, find the three vertical dots on the top-right corner:
Tap it to reveal "Settings"
This will reveal a new window showing you the email being used.
(Note that we covered it in the screenshot)