1. whatever userID your using portal for login purpiose same userid is in ECC System?
2. Your SSO is LogonTicket Method or Uid/Password Method?
if Logon Ticket method: it Should be same uersID is also avlibale in ECC System and import SAP logon ticket certification into the ECC System
for import the certification please refer this document:SSO
if Uerid/Password Method: you should do userMapping in Portal under "User Admin"