If yes, then the problem is, instead of landing in Device Firmware Upgrade (DFU) mode you are ending up in Recovery mode.
Now, if you can get back to DFU mode that will solve your problem. Once the iPhone is in (DFU) mode, you can follow the following process to downgrade your iPhone.
ProcedureWhen the iPhone is in DFU mode, this mode will bypass the operating system that is currently installed. This process aids in downgrading or upgrading your Operating System in iPhone. Remember, DFU mode is different from the usual Recovery mode.
The difference between the two modes can be spotted below:
Here, you can see there is no backlight and the screen is blank and black. This indicates that the phone is in DFU mode.
In the figure below, you can see a cable connecting to iTunes icon on the screen. This indicates that the screen is in Recovery Mode.
If there is something on the screen, then that means you are not in the DFU mode.
You have a process that can load the iPhone in DFU mode. Check out that process here:
- First, you need to plug in your iPhone to your Computer.
- Now, turn your phone off.
- Now, for the span of 10 seconds precisely, hold the Power and Home buttons together.
- Now, continue holding the home button and release the power button.
- This has to be done until you hear a beep sound from your Computer
- Now you should see a blank screen sans any display. This indicates that you are in DFU mode.
- Now your restore should work accurately.
So, next time if you have trouble with Error 21, then try restoring the iPhone with this process.