Unable to 'Change Data Source' on Pivot Table
 

I have a pivot table that I've updated each month for a few years and now I get and error message when I try to 'Change the Data Source":

"The PivotTable field name is not valid. To create a PivotTable report 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."

I have not changed anything on this report, and don't understand why this message pops up and prevents me from 'Changing the Data Source' which is merely adding a few more rows!!!

If anyone can shed some light on this I'd appreciate learning how to fix this.

Thank you,

Rob