If you received an error message during sign-up stating that your email/phone is already registered, that means that there is an active Venmo account that has already signed up with the phone number or email address you attempted to sign up with.
If you have never signed up for Venmo, please contact our support team directly so we can help out. Please be sure to include the email address/phone number that you're trying to use to expedite the process.
Note: Losing service or stopping sign-up midway through signup in the past can also cause accounts to be created by mistake.