That output may not be an error at all. An object with replies, writeControl.requiredRevisionId and documentId is exactly what the Google Docs API returns after a successful batchUpdate, which is what the step that writes text into a document calls. An empty entry in replies just means the request (inserting text, for example) had nothing to report back.
So check two things. First, open the document with that documentId in Google Docs; the text is probably there, just not where you expected, for example in My Drive instead of your target folder. Second, if a later step treats this output as a failure, map the documentId field rather than the whole object.
If the document really is empty, the usual cause is a create step followed by a write step that points at a different document id, so map the id from the create step’s output into the write step instead of pasting one by hand.