It is a simple syntax error: greater-equal is >= (not => as you wrote).
Would you mind, please, to check your forms with the online validator during development (and preferably, also before posting). It would have marked the following error:
(You get the same error if you try to deploy your form in KoboToolbox. It’s even a bit strange that you can Preview in KoboToolbox and Formbuilder without error message,)
In case you use the Formbuilder for development, you can easily download the form as XLSForm and upload it into the online validator for testing. (You can also use the Preview function there,)
To locate a problem, it is often helpful to strip down an expression (or a form) in simple elements, e.g. here just: . . => 12, which will already indicate the problem.
