Hi Mohamed914,
Welcome to Apple Support Communities. We understand you're unable to install iOS 14.8.1 on your iPhone. We can help point you in the right direction.
We suggest to remove and re-download the update on your iPhone. This may help to resolve the issue you're experiencing. For the appropriate steps, please follow the section in this article that says, "Remove and download the update again": If your iPhone, iPad or iPod touch won’t update
If that doesn't help, you'll want to connect your iPhone to a computer and complete the update. Here's two articles with detailed steps:
- Update your iPhone, iPad, or iPod touch with the Finder
- Update your iPhone, iPad, or iPod touch with iTunes
Please keep us posted with the results.
Best regards.