LE Default on Sales Order based on EFF value

Hi Team

We are trying to default a legal entity on the Sales Order using Post Transformation Rules. In our client, one Business Unit has multiple Legal Entities tagged to it. We have created an EFF at Sales Order Header where we are capturing the Legal Entity ID.

And we are trying to default this LE ID on the Sales Order using the Post Transformation Rules. But we are encountering the error:

For input string: "HeaderEffContext.legalEntityId". Any leads on this?

Thanks