Hello, hawary89!
From your post, it sounds like you are trying to transfer data saved in iCloud to a new device. We’re glad that you reached out to the Apple Support Communities. We want to try and help you find the answers you need.
To verify, what exactly happens when you try to transfer the data from iCloud to the iPhone 12? Do you see an error message? If so, what does the error say? If needed, do you still have access to the older iPhone with all the data and information still stored on it?
Also, are you following these steps when you attempt to Restore your iPhone, iPad, or iPod touch from a backup in iCloud?
Once more information is posted, others in the Apple Support Communities may be able to offer further support.
Take care!