Unable to update iPhone 8

Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• You have an active cell signal (You need both WiFi and cellular service to perform the upgrade).

• Are not using a VPN connection.

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 10 gigs of storage available.

Click on this link on what to do if you can't update your phone -

https://support.apple.com/en-gb/HT201435

To update your iPhone, you need an internet connection, enough storage, and connected to a power source. Your iPhone will support iOS software up to iOS 16, so keep that in mind.

• Enough storage on your iPhone

Make sure you have enough storage on your iPhone to update, if you don’t have enough storage, your iPhone will try to clear up storage that it can redownload after the update. You can make room for the update by removing content and apps on your device that you don't use. Go to Settings > General > [Device name] Storage.

• Download takes a long time

To update your iPhone, you will need an internet connection. To improve the speed of the download, avoid downloading other content and use a Wi-Fi network.

If your iPhone or iPad won't update - Apple Support

Your tag line says you have an iPhone 8. You can update an iPhone 8 to the latest version of iOS 16. You cannot update it to iOS 17.