Pleas provide download link for iOS 12

The iPhone 7 can not be updated to iOS 12. Any update will be to the most recent, compatible version. For the iPhone 7 that will be iOS 14.3. You may need to use iTunes on your computer to complete the update.

Could you post a screenshot of what error message you are seeing when you attempt an update using iTunes.

Do note that because you are at iOS 11, you will need to use iTunes on your computer to complete the update.

See: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad or iPod touch won’t update - Apple Support

And what happens when you attempt to update?

Any error messages? If so, what are those messages?

See: Updating iOS: common issues and solutions - Apple Community

Is the device charged?

Connected to the charge circuit?

Connected to WiFi?

Ever used a beta version on the device?

Is iTunes on your computer up to date?