I want to upgrade my iPhone 8 to iOS 15.0

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You cannot update your iPhone 8 to iOS 15. When updating, you are only allowed to update to the most current iOS, which in your case would be 16.3.1. To update, select Support > General > Software Update. If your device is eligible to update, you will be prompted to do so. You can read more about it here:

Update your iPhone or iPad - Apple Support

Is there an issue?

Settings > General > Software Update is more likely to work.

You can only update to 16.3.1.