Sorting Pivot Table for Percent of Change

Hello, I work for a non-profit and am trying to calculate % of change for year over year donations, per donor in a Pivot Table.

I am trying to gather information on donors and analyze whether previous donor remained active this year, and track the change in their giving. A pivot table format is helpful for drilling down on transactions, for follow-up... However, I keep getting an error when I try to sort the "% of difference" column.

Is there another way I can generate this information to make it sortable? Any other ways to summarize change in annual giving?