You've put "as existing" in the wrong place.
When Setting up Set Field, there are two Specify buttons that must be clicked. To get Set Field [Table::Field ; Expression], add set field to your script and click the first button (specify target field). Select Table::Field from the list of fields. Do not click the specify button next to the repetition box. Click OK to close this dialog box. Now click the lower specify button (calculated result) and create the expression to the right of the semicolon (;). Do not try to type in the semicolon.
But I'd use a conditional format with "As Existing" placed on the layout as layout text. The conditional format would be set to hide/reveal the text by setting the font size to 120+ points to make it invisible when the text does not apply.