How to edit standard fields(whether its enum , or string ) of purchtable form on status open order(BackOrder)

Im trying to edit the standard filed on the PurchTable form using active method
[ExtensionOf(FormDataSourceStr(PurchTable,PurchLine))]
final class PurchTable_Form_Extension
{
public int active()
{
int ret = next active();
PurchLine purchLine = this.cursor();
PurchTable purchTable = this.cursor();
if(purchTable.PurchStatus == PurchStatus::Backorder)
{
this.formRun().design().controlName(‘PurchLine_ItemName’).allowEdit(true);
this.formRun().design().controlName(‘buttonUpdateReceiptsList’).enabled(false);
}
else
{
this.formRun().design().controlName(‘PurchLine_ItemName’).allowEdit(true);
}
return ret;
}
}

my code is running fine for customize field but it did’nt work for standard field

im trying to edit the product name here using its controller name but this above code is not working for the standard field