How to understand where the source of the 400 error is?

Question

Django

The back is written in Django, the front is written in react. When I send a POST request, I get a 400 error.
In the uwsgi logs I find

Bad Request: /api/v1/vendors/upload_excel_rfi/20R6/417/0/
[pid: 9088|app: 0|req: 551/925] 93.84.150.241 () {56 vars in 1317 bytes} [Mon May 25 13:44:29 2020] POST /api/v1/vendors/upload_excel_rfi/20R6/417/0/ => generated 48 bytes in 373 msecs (HTTP/1.1 400) 6 headers in 185 bytes (2 switches on core 0)

In the browser developer console in the Console tab

xhr.js:178 POST https://api.spend-matters.qa.sumatosoft.work/api/v1/vendors/upload_excel_rfi/20R6/417/0/ 400 (Bad Request)

Clicking on xhr.jsmoves to a line of code with an erroneous status

// Send the request
    request.send(requestData);
  });
};

Failed to load resource: the server responded with a status of 400 (Bad Request)

I have no idea where to look for the source of the problem. Is it back or front?