Prepare write and execute write requests are the be handled at the BLE stack event handler to support long writes. If not done, the BLE stack assumes that long writes are not supported and returns the error that you mentioned.
See this example project - https://github.com/yourskp/BLE/tree/master/Long%20Characteristic%20Read%20Write
Thanks for the answer. That what I had assumed. Now confirmed.
Thanks & Regards,