What stopped you changing the port in SQL manager - and are you sure changing the port will work? If you have a firewall generally blocking the connection between the machines, no port will function.
Can you ping the "specified sql server" from your EPO machine?
A common cause of this message is that the SQL Browser service is not running on the SQL server. (This is the service that provides dynamic port support for SQL, using UDP 1434 by default.)
Check to see if this is running in your environment: if not, either start it, or if that's not possible then (as the error message says) in the next step of the install you can manually enter the TCP port for the SQL instance.
Thanks Joe & SafeBoot for your answers.
when i got the error i started the SQL browser , but i got the same error.
i re ran the setup , now its fine.