The iPhone error 21 pops up when you try to restore your Apple device. It is a common issue faced by most of the iPhone users. The first thought which strikes your mind when you see this error is that 'Product fault', 'Software issue', 'Manufacturer's mistake', etc. In this article, you can analyse the possible reasons behind this error and optimum solution to handle it. It is high time to quick fix iPhone error 21 effortlessly. Follow the below guidelines carefully and resolve the issue in no time.