please help my iPhone update 12.4 is stuck on my iPhone 8

iOS 12.4 is a beta firmware version.

As such Apple Betas are confidential and cannot be discussed in the public forums, like the Apple Support Communities, as per the terms you agreed to when joining. If you read the agreement carefully, you should know that any issues can be submitted via the Feedback Assistant app that was installed along with the Beta.

Ref: Apple Beta Software Program - Apple Beta

How do I provide my feedback to Apple?

The iOS and macOS public betas come with the built-in Feedback Assistant app, which can be opened from the Home screen on your iOS device or from the Dock on your Mac. The Feedback Assistant app is also available from the help menu of any app by selecting Send Feedback. If you’re running the tvOS public beta, you can submit

feedback through the Feedback Assistant app on an enrolled iOS device. When you experience an issue or something does not work as expected, send your feedback directly to Apple with Feedback Assistant.

Is the public beta software confidential?

Yes, the public beta software is Apple confidential information. Don’t install the public beta software on any systems you don't directly control or that you share with others. Don’t blog, post screen shots, tweet, or publicly post information about the public beta software, and don't discuss the public beta software with or demonstrate it to others who are not in the Apple Beta Software Program. If Apple has publicly disclosed technical information about the public beta software, it is no longer considered confidential.