Hello lu_rawl,
Welcome to the Apple Support Communities! We know how important it is to be able to use your iPhone 12 MagSafe case. To be sure we're on the same page, you're not seeing any animation appear on the screen when you place the iPhone in the case.
If this is correct, and if this happens to be an iPhone 12 Leather Sleeve with MagSafe, be sure that you have installed the latest updates to iOS 14. This article will help you to ensure that you have a backup of your iPhone data before updating. Having a backup of your data is like having an insurance policy, you don’t plan on using it, but it’s great to have if you need it: How to back up your iPhone, iPad, and iPod touch.
Once backed up, you'll want to install any updates. This article provides the steps to follow to update wirelessly as well as how to update using a computer: Update your iPhone, iPad, or iPod touch.
If you find that there is still no animation on the screen following any updates, you can Contact Apple Support for more assistance.
All the best.