Error message when viewing the form

I am trying to preview the form of the questionnaire but I get this error message:

FormLogicError: Could not evaluate: Calculate the age, message: not sure how to handle expression called on nodeset that doesn’t start with a ‘/’: theage{“stack”:[{“t”:“root”,“tokens”:[{“t”:“arr”,“v”:}]}],“cur”:{“v”:“theage”}}

How can I solve this problem?

Welcome to the community, @ariannavivoli! Have you validated your XLSForm through this online validator? The online validator should help you identify the syntax issues that is affecting your form preview.