Issue with connecting to ReplicationServer with sql authentication

I am trying to use powershell to connect to ReplicationServer and backup replication.The SQL Server Editition is 2008 SP2. It works fine with windows authentication but when I try to connect with sql authentication it shows error New-Object : Cannot find an overload for "ReplicationServer" and the argument count: "1". At .....\Experiment.ps1:34 char:74 + [Microsoft.SqlServer.Replication.ReplicationServer]$var_server=new-object