Hi Reen,
Crystal Reports wants both the 'True' and 'False' results to have the same data type. In your example, the 'True' result is a character, but the 'False' result is a number. This is an error as far as Crystal Reports is concerned.
There is a way to do this without even having to create a formula field. Assuming you have the 'Qty on Sales Order' field already in your report, right-click on it and pick 'Format Field' from the pop-up menu. From the Number tab, click the Customize button below all the pre-defined number formats.
On this 'Custom style' screen at the bottom right side, there is a selection for 'Show zero values as', and a dash is one of the available choices. Pick this and click OK to save it. Now when you print the report, zeroes are replaced by dashes.
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Regards,
Softrak Tech Support