Hi,
Have you Define the SAP ECC system and Alias in the portal system landscape. Try to do connection test in system landscape.
Does the user with which you are trying have proper access in the backend.(Check he is not locked in the backend or doenot have valid date.)
As mentioned earlier check SSO.
If the portal and the backend are on different system check that the post is open and you can use telnet command to connect the backend.
Thanks
Rihsi Abrol