Excel calculated field greyed out
When using a Pivot Table you may discover the Excel calculated field greyed out. One of the main reasons for this is that the underlying data is from an OLAP source. Another reason for this in Excel 2010 and above is that the Pivot Table you are looking at is actually a PowerPivot Pivot Table (actually the same reason but you could have underlying data you did not know you had).
Related: Learn how to use PowerPivot