What about the parameter? Does it exist in the model?
In your code you get a parameter DESCRICAO_PAR, read the value of this parameter. Then write this value back to parameter. If the model don't have this parameter, then you will get an error.
Dim paramOwner As IpfcParameterOwner
Set paramOwner = session.CurrentModel
Dim parameter As IpfcParameter
Set parameter = paramOwner.GetParam("DESCRICAO_PAR")
Dim modelItem As CMpfcModelItem
Set modelItem = New CMpfcModelItem
Dim paramValue As IpfcParamValue
Set paramValue = modelItem.CreateStringParamValue("TEST")
parameter.SetScaledValue paramValue, Nothing