In this case it is a server side error, not a client error as previously reported. Because the error occurred on a dblink, the error is at the db server.
You are querying a table in a remote database. The Emp@Rept means you are querying table Emp in the data source identified by dblink Rept. The names are not case sensitive. Rept is the name of a dblink that points to an entry in file tnsnames.ora on the database host, and has nothing to do with the client.
First, log into the database using sqlplus or OEM and look at the dblink to see what it points to. The properties might include a TNS string, username, and a password.
Second, log into the database server where the error occurred as user oracle (or who ever owns the database home) and look at file $ORACLE_HOME/network/admin/tnsnames.ora. You must find an entry that corresponds to the dblink's properties.
Third, use the "tnsping" utility to test the tnsnames.ora entry.
If you have global_names=true then the name of the dblink "Rept" must match the name of the entry in tnsnames.ora.
If you have file sqlnet.ora (in the same directory as tnsnames.ora) look for the parameter sqlnet.default_domain. When the dblink is accessed Oracle appends the domain to it, so the entry in tnsnames.ora would be of the form Rept.yourdomain.