error: Field 'CostPerUnit' not found

[ELI4]

Hi everyonw. I am new in qlik. I don't know where is the problem. Can you help me?

Qualify *;
Unqualify ItemID;

CampaingID_Map:
Mapping LOAD
    CampaignID,
    Name as NameofCampaign
FROM [lib://ExtractDB/Campaigns.qvd]
(qvd);


Salesnews:
Load
    *;
LOAD 
    (Sales - CostPerUnit) as Benefit;
LOAD
    ItemID,
    Date,
    CampaignID,
    DocumentNumber,
    StoreHanut as StoreCode,
    "Time",
    SalesmanID,
    if( Quantity>= 0, 'Sold', 'Return' ) as Status,
    ApplyMap('CampaingID_Map', CampaignID , 'No Campaign' ) as NameofCampaign,
    TotalPriceBeforVat,
    Quantity,
    (TotalPriceBeforVat*Quantity*1.17) as Sales
FROM [lib://ExtractDB/Salesnews.qvd] (qvd);

left Join

Stock:
LOAD
    ItemID,
    Cost as CostPerUnit
FROM [lib://ExtractDB/Stock.qvd]
(qvd);

Unqualify *;

I want to add a new field call Benefit.(Sales-cost per unit) But Doesn't work!

thanks

[Or]

You've used Qualify *, which means all fields will have the table name added preceding, e.g. Stock.CostPerUnit

For details, see https://help.qlik.com/en-US/sense/May2021/Subsystems/Hub/Content/Sense_Hub/Scripting/ScriptRegularSt...