An email address can only ever be used for one user account. If you are ever creating a user account and are met with this message it means that the email address used is already associated with another account.
How to free up the email address
If the email address that's getting this error needs to be used for a different account then you are able to amend it to allow it to be used.
First, you will need to find the existing account using that email address and open it up. On the right hand side, select change email address and amend the email address in some way so that it can be used again.
Where possible, it is best to change the email address to something that is not a real email address to prevent accidentally sending information out to it.