Unfortunately you cannot use VLOOKUP within a pivot table calculated field.
You can solve this by either placing the VLOOKUP outside the pivot table or by placing the VLOOKUP within the original source data that is used by the pivot table.
You can find a lot more about calculated fields in the video lesson: Expert Skills Lesson 5-17 Add a calculated field to a pivot table.
This lesson also shows how it can be easier to perform calculations within your source data rather than by using calculated fields.
You can also find an in-depth look at calculated items in the video lesson: Expert Skills Lesson 5-18 Add a calculated item to a pivot table.
I hope this helps, but please feel free to reply if you need any further assistance.
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