Kindly note that it means the email address you're trying to add, has been registered as an account already. You may create a new account and if the issue persists try using a new email address through the below steps:
Go to the Google Account creation page.
Follow the steps on the screen to set up your account.
Use the account you created to sign in to Gmail.
If the problem persists, please send us an Error Report by following the steps below.
Head to the Members app > Get Help > Error Report > Tap 'OK' to send System Log Data > Select the symptom, and include as much detail as possible.
To ensure your logs accurately reflect the issue you're experiencing, we'd like you to submit your report within 5 minutes of encountering the problem. Our engineers will then investigate this and respond to you with a solution via the Members app.