使用其他表中的数据转换表中的货币

I am trying to covert the currencies within a table of sales which uses Substitute IDs of all the related tables (and places them within the relevant column, so: SubstituteItemKey within the Item table is equal to the ItemID within mainsales) and specifies the transaction values. The Snippet of code which fails is:I am trying to covert the currencies within a t