Error defining picklist

Hi,

I'm getting an error trying to define a picklist...code is below:

    must have Office
    (
        type = text
        width = 200px
        on user input
        {
            if (count(Order_form[Office_Name == input.Office && Received_Impressions && Sent_to_HK == false]) == 0)
            {
                clear Order_List;
            }
            else
            {
                for each y in Order_form [((Office_Name == input.Office && Received_Impressions) && Sent_to_HK == false)] sort by Form_No
                {
                    Order_List:ui.add(y.Form_No);
                }
            }
        }
    )
    Order_List
    (
        displayname = "Order List"
        type = checkboxes
        values = Order_List.Form_No
    )

If it's of any relevance, the field Office is populated in the Onload statement - see here:

        on load
        {
            input.Date_Sent = zoho.currentdate;
            O = User_Table [User_Name == zoho.loginuser];
            input.Office = O.Office;
            hide Office;
        }

I'm getting an error:

Error modifying the page
Error at line:39
Form Order_List does not exist in this application

What am I missing here?

Thanks

Anna