Hi WTH2021,
Welcome to Apple Support Communities. We understand an error message appears that the data transfer cannot be completed to a new iPhone 12 Pro. We'll be happy to help.
If you're using the method of, Quick Start you'll want to keep your previous iPhone near the new iPhone 12 Pro through the data transfer. Please make sure to keep both devices connected to a power source and until the data migration completes. Transfer times can vary based on factors such as the connection type that you're using, network conditions, and the amount of data being transferred: Use Quick Start to transfer data to a new iPhone, iPad, or iPod touch
For issues with restoring an iCloud backup or backup from a computer, we suggest to follow these two articles: If you can't back up to iCloud or restore from iCloud backup failed and If you can't back up your iOS or iPadOS device to a computer or restore it from a backup
Please let us know if that helps.
Kind regards.