Somehow, something has been changed on my masterlist and I am not that proficient at Excel enough to figure out what has changed.

The window:
The PivotTable field name is not valid. To create a PivotTable, you must use data that is organized as a list with labeled columns. If you are changing the name of a PivotTable field, you must type a new name for the field.

Can anybody offer any advice as to where to start looking?