The issue is you are missing an '=' sign! 🙂 The message in the Edit Expression dialog is trying to tell you as much with the 'Garbage after expression: 'error' message too.
Simply change your expression to =SUM(1)error, and that should change things. I tested things in 12.30 SR3, just FYI.
I did notice the Help does not say to do this, so I can understand the confusion here, I will see if I can have someone have a look at that as well.
Regards, Brett
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