An "[insert email here] is already in use in another account on Google" error means your Google email is already connected to another login using an alternate login method. This may be Facebook if you are on an Android device, or Facebook or Apple ID on an iOS device. Here is how to proceed:
- Return to the login page.
- Select the Other Options button to log in using another login option.
- Select Facebook or Apple ID login.
- Log in using the email in question.