Update Error while installing iOS 18 to iPhone 15 Pro

I have been trying to install ios 18 update on my iphone 15 pro but a message box pops up saying there is an error. It says: “unable to install update. an error occurred. an error occurred installing ios 18”. does anyone know what could be the reason? as far as i know iphone 15 pro supports ios 18.

[Re-Titled by Moderator]