Follow these steps in order to re-install Sophos Home using your existing account:
Reinstall Sophos Home (using an existing account)  

1- Open your Sophos Home Dashboard from your antivirus:   

Mac: Click on the Sophos Home shield, click on … , click on Manage Devices 

Windows: Click on the Sophos Home shield on your system tray, click on Dashboard

2- Once at your Sophos Home Dashboard , if you are a new user, you will be automatically prompted to Create an account

3 - Follow the steps to enter an email address for the account and validate it
(see Email address confirmation for additional details) 

Note: If you do not receive the confirmation email, it was likely intercepted by a spam/junk mail filter. Check your spam/junk mail AND check your junk mail settings. You can also whitelist
no-reply@mail.home.sophos.com per your email providers' settings.

4 - Your Sophos Home Dashboard is now tied to your account, which will allow you to protect additional devices, re-install on your computer and, for Premium users, access support.

5 - To access your Sophos Home Dashboard, either use the Manage options in the antivirus, or access https://my.sophos.com and log in. 

This message indicates that the email address is already in use with a Sophos Home account.

There are 2 options to address this, follow the one that applies best to your case: 
Option 1 - [Requires re-installation] Log in to your existing account and re-protect your computers via that dashboard. See How to reinstall Sophos Home 

Option 2- Create a new account using a different email address to keep using the current dashboard you are currently trying to access. 

Please visit MFA - How to enable Multi-Factor authentication for additional security steps to help secure your Sophos Home account.