I created a relationship using ProductID and tried your expression. But it gave the following error.
Please refer the below statement I found from web.
"The RELATED function requires that a relationship exists between the current table and the table with related information. You specify the column that contains the data that you want, and the function follows an existing many-to-one relationship to fetch the value from the specified column in the related table."
Therefore, I can get the 'Target' value from 'Target Table' to 'Sales Table' using RELATED function. But, I want to fill 'Cumulative Ach Qty' column in 'Target Table' when 'Sales Table' is updated. Is there any suggestion please?