Please use an email that is not associated with any existing Coinbase accounts. You cannot sign up for Prime using an email associated with one of the following:
Email used for Coinbase.com account.
Email used for any Institutional account (Exchange, Custody, Prime). If you already have a Prime account, you will need to use a different email to create a new account.
If you only have one email account, you may add “+prime” at the end of your email address as a workaround (i.e. firstname.lastname@example.org). You will still be able to receive emails in your regular inbox. However, please note that this workaround only works for Gmail accounts.