The error message you have encountered is caused from the configuration you have set up.
In Rest Receiver channel in tab ‘REST URL’ you have configured ‘Pattern Variable Replacement’ with an XPath expression. The problem is that when the REST Adapter try to search for this XPath expression, it already has payload in CSV format, which try to parse to XML to find the required XPath. And here it fails.
This means that you cannot extract any XPath or JSON expression because the structure of the payload is CSV and the REST adapter does not supposed to work with this kind of structure.
The only way in this case to get this ‘sessionId’ element value is to pass it to the REST Receiver channel as Dynamic attribute of the XI message/payload/.