If you're receiving an error message that says your email is already in use, you must have accidentally created an account using this address in the past.
There are two ways to make a Mistplay account: Through Google, or Facebook.
If you accidentally created an account in the past using Google Signup, you will not be able to add the same gmail address to another account you've created using Facebook login.
You can either:
1) Add a different email address to the account you're currently using
2) Sign out of the account you're using, and log in to the other account that has your email address on it. Once logged in, visit the edit profile page and change the email to something else (it doesn't need to be a real address.) Then, you can return to your main account and add the email you wanted there.
If you require further assistance, please contact our support team by selecting "submit a request" at the top of this page.