If the 7+ has iOS 14.6 then the 12Pro needs to be updated to 14.6. To do this set up the 12Pro as NEW, then go to Settings/General/Software Update and update it. After you have done that go to Settings/General/Reset - Erase all content & Settings. The phone should go back to the Hello screen after a few minutes, and you can start the transfer over.
If it doesn’t reset to the Hello screen after 10 minutes, do a forced reset:
- Press and quickly release VOLUME UP
- Press and quickly release VOLUME DOWN
- Hold the SIDE button until an Apple logo appears (15-30 seconds)
The operative word here is QUICKLY
There is one other less likely, but possible problem. If you have a beta version on the 7+ you will have to install the beta version profile on the 12Pro and update it to that also.
Then follow these instructions→Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support