Error 6: Operation failed when doing OTA in iOS

**Describe the bug**
When operating OTA update in iOS using nrfConnect (the official APP), we found an issue like this:
xx means the ID

peripheral.writeValue(0x0201, for: xx, type: .withResponse)
Data written to xx
Notification received from xx value (0x): 100206
Response (Op Code = 2, Status = 6) received
Error 6: Operation failed
Writing to characteristic xx...
peripheral.writeValue(0x06, for: xx, type: .withResponse)
Notification received from xx, value (0x): 100206
Response (Op Code = 2, Status = 6) received

**Set up (mandatory)**
- **APP** : nrfConnect, official APP
- **Board** : Feather nRF52832
- **sketch**: blincky_ota example, see: https://github.com/adafruit/Adafruit_nRF52_Arduino/tree/master/libraries/Bluefruit52Lib/examples/Peripheral/blinky_ota
- **reprodce the issues**: after flushing the blinky_ota.zip, connect it in nrfConnect, and then carry on OTA UPLoad, it will fail.

Start with the Peripheral/blinky_ota example and reproduce the OTA upload on a Feather nRF52832 using the nrfConnect iOS app. Inspect the reported Op Code 2, Status 6 response sequence and determine what prevents the upload from completing; done means the documented setup completes OTA successfully.

Written by the indexing model from the issue text.