Hi there ytzabar,
Thanks for joining us in Apple Support Communities! We understand an NFC tag isn't scanning as expected on your iPhone 12. We're glad you're here, and we can provide some guidance.
We'd like to start by clarifying that you won't see the option to enable NFC scanning in Control Center on iPhone 12 because it's supported automatically.
"Supported automatically on iPhone XS, iPhone XS Max, iPhone XR, iPhone 11, iPhone 11 Pro, iPhone 11 Pro Max, iPhone 12, iPhone 12 mini, iPhone 12 Pro, iPhone 12 Pro Max, and iPhone SE (2nd generation). Supported after turning on NFC scan mode from Control Center on iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, and iPhone X." Have a look at the number 14 in the footnotes at the bottom of this link:
iOS 14 - Apple
With that said, if the NFC tag isn't scanning automatically when using the associated third-party app, you'll want to follow all the steps in this support article, including ensuring your iPhone is updated to iOS 14.5 and installing any updates available in the App Store:
If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open
If the issue persists after working through those steps, you'll want to connect with the app developer for more help. This link will help you get started:
How to contact an app developer
We hope this information is helpful.
Take care.