patchDocument - input.prepForXml Error

[Blair-24]

Just wondering if its a limitation or I'm doing something wrong. Trying to patch an existing document with an array of objects that I want to format into ultimately repeating sections for each object. This is my code at present (plan is to add more, e.g. tables etc):

But I get this error:

TypeError: Cannot read properties of undefined (reading 'prepForXml')

[Blair-24]

I've built out a bit more of the code to show where I'm heading. This is the item I'm returning to the children item within the patchDocument:

But still get the same error.