Are those the exact names of the debit/credit fields on your side? I just put generic names but the [Debit] field should match the column name of your Debit column - for example, if it's named [debits] then make sure it reflects as such in the Formula. Same thing with Credits as well!
In addition, make sure the fields are both numeric and not strings - typically these are stored as Double but could be stored as FixedDecimal or Float. You can check the Metadata in the Results pane quickly or put down a Select tool in between the Union and the Filter.
If you want more direct help on the workflow, go ahead and "Export Workflow" under Options - this will package the workflow and any relevant data together into a .yxzp file. Make sure to redact any sensitive data!