IOS update to 6S plus
trying to update IOS 15.3 on 6S plus with computer. Keeps getting the message "iTunes could not back up the iPhone because the iPhone is disconnected. Please disconnect and reconnect the iPhone, then try again. In fact, the phone was connected the entire time to the computer with an original apple wire. I followed instruction to restart both the computer and phone but still get the same message after.
iPhone 6s Plus, iOS 15