Header content type should be URL encoded --header 'Content-Type: application/x-www-form-urlencoded' and form input data should be in the same format as given in the examples.
I found FAQ below to be helpful—any other helpful links? https://kite.trade/forum/discussion/8019/faqs-on-pykiteconnect-specific-to-python-client#orders
I just placed a MARKET order from the kite publisher and COMPLETE status is received absolutely fine on the postback URL. You need to inspect your local postback setup. Go through postback FAQs here.