Syntax Error: "Target is Missing"

Hi, all

I am trying to get my user input interface to display in Design mode so I can re-map inputs to changed fields in my CDTs.

I am unable to open it in Design mode, only in Expression Mode.

I am seeing the triangle with the exclamation mark in the margin. When hovering over this I get the following error:

Expression evaluation error: Syntax error. Details: Expression evaluation error at function a!floatingPointField parameter 8 [line 239]: The Target is missing.

Here is the relevant section of code:

The rule input is valid.

Any ideas?

Thanks

Stephen