Zoho Writer Sign API error R8020

Zoho Writer Sign API error R8020

Hello, 

I am trying to send to sign a document which has been merged and stored  in Zoho WorkDrive using the Merge and Store Writer API. When I run the Sign API as follows I get an "internal server error":

  1. url = "https://zohoapis.eu/writer/api/v1/documents/" + doc_id + "/sign";
  2. documentObj = invokeurl
  3. [
  4. url :url
  5. type :POST
  6. files:paramList
  7. connection:"mail_merge"
  8. ];
  9. info documentObj;

The error is:

  1. {"error":{"response_generated_time":1692347051131,"documentation_url":"https://www.zoho.eu/writer/help/api/v1/errors.html","message":"An Internal Server Error occurred while processing this request.","request_url":"https://zohoapis.eu/writer/api/v1/documents/c3o9aec94b75ada5a4b6db85ff6faa6bde140/sign","errorcode":"R8020"}}

Could you please let me know how to fix this?

Thanks in advance,
Giulia