In your First line
If rdbPO.Checked = True And ddl_PO_Cod.SelectedValue.ToString <> -1 Then
write condtion like this
If rdbPO.Checked = True And ddl_PO_Cod.SelectedValue.ToString <> "-1" Then
OR
If rdbPO.Checked = True And Convert.ToInt32(ddl_PO_Cod.SelectedValue) <> -1 Then
Like this..
When you are comparing two strings then strings must be in " "(i.e double inverted comma)
Or if you sure about your selected value is Integer then use
Convert.ToInt32(combobox.SelectedValue)