This formula is not working its giving me this error:= 'Sum of WriteOffTotal' / 'Sum of InvoiceAmount'

AutoSave Off ) Session / Final Project (4) v Search e Home Insert Draw Page Layout Formulas Data Review View Automate Help Acrobat PivotTable Analyze Design ot Table Name: Active Field: - Group Selection Insert Calculated Field ? X rotTable3 Sum of WriteOffTo Show Ungroup Insert Insert Filter Refresh C Recommended Fie Add Options Field Settings Details 7 Group Field Slicer Timeline Connections Name: Write Off % PivotTables Lis PivotTable Active Field Group Filter D Formula: = 'Sum of WriteOffTotal' /'Sum of InvoiceAmount' Delete Tools SECURITY WARNING Automatic update of links has been disabled Enable Content Fields: v Sum of WriteOffTotal Invoice ID Sales Order ID A B C D E InvoiceAmount Entered By Table Fields Customer ID Cutomer Name Write OffTotal fields to add to report: ow Labels T Sum of InvoiceAmount Sum of WriteOffTotal Write Off % ander Davies 1978.08 862.26 43.5 Insert Field asmin Terrell 2890.78 621.16 21.4 1611.17 670.86 41.6 OK asmin Smith Close stomer ID asmin Sellers 1532.69 1428 Cutomer Name amilet Robertson 1003.03 710 Microsoft Excel X WriteOffTotal avier Cortez 3173.82 763 WriteOff % ander Mcpherson 2918.51 391 Calculated item or field could not be created because another item or field of the same name exists. InvoiceDate ander Krueger 331.14 331 OInvoice Due Date OK Whitney Turner 5398.2 962 HasShipment eronica Miranda 1065.19 697.81 65.51% More Tables... aughn Vance 1398.91 1398.91 100.00% alery Pena 3968.89 550.45 13.87% Drag fields between areas belo