Why does my iOS 15.5 say error when I try to install

Try to start over.

First. Go to:

Settings -> General -> iPhone Storage

Check for the iOS Update, if you find it then tap and delete it. After that - close all your apps and restart your iPhone.

When your iPhone is turned on again, log on to a good WiFi network and connect your iPhone to a charger.

Go to Settings -> Display & Brightness -> Auto Lock and set to Never. This is to prevent the phone from going to sleep while the update is being downloaded. Remember to set the setting back after the update.

Now, go to Settings -> General -> Software Update and start downloading iOS 15.5 - just leave your phone while the update is downloaded. When the phone shows "Install" just press and let your phone update. Hopefully it shows no errors this time.

Good luck.

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